Symbolism, Imagery, Allegory
Okay, so you've heard Mr. G. use this word a lot. He likes capitalizing it, italicizing it, and doing all he can to show how super important it is to him. But what does it mean?
Gandhi defines ahimsa as "literally non-hurting, non-violence" (1.7.12). It means more than just refraining from striking your opponent; it means wishing to do them good. Yup: deciding not to push annoying Casey From Down the Street into a bus ain't good enough when it comes to being Gandhi-riffic. You'd actually have to a) not push Casey From Down the Street into a bus and b) buy him his favorite Panera Bread sandwich as a treat and hope that he lives happily ever after.
In short: it takes a big ol' mensch to pull off ahimsa. (Look at us, blending Yiddish and Sanskrit.)
Oh, and when it comes to your enemies: you definitely don't want to inflict himsa on them because that's violence or hurting (4.39.4), the opposite of ahimsa.
So, ahimsa means non-violence, but why is it so important? What's wrong with a little payback? Well, Gandhi sees ahimsa as "the only means for the realization of Truth" (Farewell.4). So, if you want truth (which Gandhi does), you have to get there via ahimsa.
It gets even harder than just being a selfless well-wisher. You need to first purify yourself because "without self-purification the observance of the law of Ahimsa must remain an empty dream" (Farewell.6). Ahimsa implies caring and compassion for all life…and if you're not striving to care for all life—including your enemies' lives and your own life—Gandhi says you're not really seeking truth.
For more ahimsa-y goodness, check out the "Philosophical Viewpoints: Non-violence" theme.