From Maggid Meisharim, parashat Ki Tisa, Rabbi Caro's recording of the teachings he heard from his angelic mentor (see his Biography).
"If I am not for myself, who will be for me?
And if I am for myself, what am I" (Avot 1:14).
"If I am not for myself, who will be for me" means if I do not grasp hold of the "I," namely malchut [which is called in Hebrew "ani"], who will be for me -- that is, to which of the supernal sefirot can I dedicate myself?
Malchut represents the authority of G‑d, and dedicating oneself to malchut means accepting upon oneself the Yoke of Heaven. If one does not accept the Yoke of Heaven, what holiness is there in emulating any of the sefirot?
"And if I am for myself, what am I" means when I grasp the "I," malchut, and I take it "for myself," for the lower malchut [malchut of the lowest world, Asiya, namely, the human individual, in this case "myself,"] as it says in the verse, "Bone of my bone, flesh of my flesh" [a play on words in that the Hebrew word for "bone" shares the same root as that for "myself"] (Gen. 2:23) - then "what [in Hebrew, Mah] am I?" [This can be read as "I am Mah,"] meaning that at that moment I join together the Name Yud-Hei-Vav-Hei expanded to the numerical value of Mah  with "ani" ["I"], malchut.
In other words, when I take upon myself the Yoke of Heaven (malchut) and make it part of me, there is revealed upon me the Ineffable Name, whose transcendent, indefinable quality is alluded to in the word "what" -- ma'h.