Ahimsa is the first yamas of Ashtanga Yoga. OK so for some of you I just said 'Wha wha wha wha wha" just like the teacher in Peanuts cartoons.
Simply...Ahimsa means non-harming. Doing no harm not just to others, and all of life's little creatures, but also self.
So for me there are two things to remember in everything in life. 'Satya' and 'Ahimsa'. Truth and doing no harm.
In yoga these are especially important things to remember. Be honest with yourself and your instructor. And never do anything that hurts or that you know will harm you.