Amino Acids are the building blocks of protein. Amino Acid supplements can give you energy, but more importantly they help prevent your body from going catabolic. (When your body is catabolic it breaks down muscle because it doesn't have enough nutrients).
Amino Acid supplements are generally taken during your workout, to give your body the nutrients it needs while you are in the gym, and immediately afterwards.
Amino Acids also help with recovery, so adding one of these supplements to your diet should leave you feeling less sore and better recovered the next day!