About this passagePirke Avot is a tractate of the Mishnah, and unusually among them it holds no law — only sayings, each attributed to a named teacher, arranged as a chain of transmission. Hillel taught in Jerusalem in the generation before the destruction of the Second Temple. Three questions, each undercutting the answer the last one invites: nobody else will do this for you; doing it only for yourself makes you nothing much; and the deliberation itself can be the delay.
He used to say, "If I am not for myself, who will be for me? But if I care for myself only, what am I? And if not now, when?"
In plainer words
If I do not look after myself, nobody is going to. But if looking after myself is all I do, what exactly am I? And if I keep putting it off — when?
A restatement in modern English, written for this collection. The passage above is the text; this is one way of putting it.