For an explanation of the methodology of this series, see the introduction.

"If there will arise among you a prophet, or a dreamer of a dream, and he gives you a sign or a wonder." (Deut. 13:2)

Peshat (basic meaning):

Rashi: "and he gives you a sign"
There are two different versions of his commentary in the traditional texts.

Version one: [a sign] in the heavens, as it is stated in the case of Gideon [who said to the angel]:"then show me a sign" (Jud. 6:17), and then it says [further],"let it be dry only upon the fleece [and upon all the ground let there be dew]." (Jud. 6:39)
Version two: in the heavens, as is written, "and they [i.e., the sun, the moon, and the stars] shall be for signs and for seasons." (Gen. 1:14)

Rashi: "or a wonder"
There are two different versions of his commentary in the traditional texts.

Version one: [a sign] on the earth
Version two: [a sign] on the earth, as is written, "If there will be dew on the fleece only, and upon all the ground, dry." (Jud. 6:39) Even so, you shall not listen to him. But if you say, "Why then does G‑d give him the power to perform a sign? [Scripture replies,] "for the Lord, your God, is testing you [whether you really love the Lord your God]." (verse 4)

Remez (hinted meaning):

Baal HaTurim: "if there should arise in your midst":
The gematria of "b’kirbecha/in your midst" is 387, equal to "This is the woman."

Derash (interpretive meaning):

Ohr HaChayim: The word navi/prophet implies that even if he has an established reputation as a true prophet, if he asks you to violate any commandment even temporarily and it involves idolatry he is guilty of death. Here the prophet seeks to legitimize himself by means of a certain sign or miracle. He tells the people that if as a result of his attempts to create a miracle, then that would be proof the G‑d had asked people to worship a certain god. Although the people may have freely decided to worship that god, the prophet is still guilty of the death penalty.
...when his instructions contravene what is written in the Torah, you must ignore him and treat him as an imposter...
Tzeror Hamor: Moses had earlier warned about abominations committed by Cananaites; you might think that these warnings were issued on the premise that you would never indulge in such things. But if a prophet would ask you to do it, you would be honor-bound to follow him. Moses has to warn the people that regardless of the potential seducer, even if he has authenticated himself with miracles, when his instructions contravene what is written in the Torah, you must ignore him and treat him as an imposter, as a false prophet.
Moses adds that just as in a human relationship love is something that can be proven only when put to the test, so loving G‑d is also a relationship that is proven only by having been successfully tested, "in order to ascertain if you really love G‑d or not"
Ramban: Scripture commands that we don't listen to he who prophesies in the name of G‑d to worship idols nor take note of the signs and wonders he will make. And it mentions the reason - because we know from the Exodus from Egypt which was an actual event—not a vision nor a mirage—that the earth is G‑d's, and He is the Creator, the One Who wills all, and the Omnipotent, and there is no god beside Him, and we further know from the revelation at Mount Sinai that, "face to face," He commanded us to walk in this way, not to worship anything at all beside Him, and that He has not given His glory to another god. Thus the prophet has spoken perversion against G‑d, Who never commanded him so, or he spoke perversion against His Glory, for it is not proper to worship another god.
Lubavitcher Rebbe: "G‑d is testing you to know whether you do in fact love G‑d"
The stronger one's faith remains even under adverse circumstances, the sooner it will become clear it was all a matter of a test. But this faith should not be merely a matter of lip service, but must have the full force of conviction.

Sod (esoteric, mystical meaning):

Zohar Va'era Midrash Neelam 106:
Rabbi opened by quoting the verse: "Now these are the nations which G‑d left, to test Israel by them..." (Jud. 3:1) Rabbi said: I have been looking into that world, and saw that the world can exist only because of righteous people who have control over the desire of their hearts. As it is written: "this he ordained in Yehosef for a testimony..." (Ps. 81:6) Rabbi Yehudah then said: Why did Joseph/Yehosef merit that high level and the kingdom? The answer is because he overcame his lust. As we have learned, the heavenly Kingdom awaits he who overcomes his lustful desires.
G‑d has created the Evil Inclination solely for the purpose of trying humanity.
As Rabbi Aha has said: G‑d has created the Evil Inclination solely for the purpose of trying humanity. And does G‑d intend on trying humanity? Yes. How do we know this? From the verse: "If there arise among you a prophet or a dreamer of dreams...And the sign or the wonder came to pass...For G‑d your G‑d tests you..." (Deut. 13:2-3)

BeRahamim LeHayyim:
What to make of dreams? The Talmud calls them a "sign/letter," and if you fail to interpret them, it is like a letter unread. Think about that. A letter sent to you from your inner Maggid/story teller, telling you something you need to know from the higher and more concealed realms. If you don't kick it around, it goes unread. And when you do, Beware! — Dreams follow their interpretation!
Got a bad dream? We Jews even have a mechanism all set up called "Amelioration of a Dream": One who has a bad dream should see 3 buddies to declare that the dream should be interpreted for the good. This is based on the concept that a dream may include a portent of things to come, as the Talmud illustrates that most indications can have either a good or a bad result. So, the sincere good wishes of the 3 listeners can bring about a dream's favorable interpretation. (Berachot 55b)

The dreamer need not tell his friends the dream, but he should have it in mind during the ritual that appears in most traditional prayer-books. It is good to fast and do teshuvah, , but not required. If the dream is very upsetting, one may even fast on Shabbat or Yom Tov. (Shulchan Aruch, Orech Hayyim 220, 286)
We are also told that our thoughts during the day prepare the way for dreams at night. Naughty thoughts > Naughty dreams.
Sounds simple, but probably has lots of truth. The above Zohar additionally reveals the danger in accepting another's dream as a prediction from G‑d. We need to be careful about accepting another's reality or belief structure hook, line, and sinker. The Torah warns about such a "dreamer", saying that it's all a test from G‑d to keep us on the straight and narrow path.

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