Branched-chain amino acids (BCAA) are essential amino acids (specifically, valine, leucine and isoleucine). They are essential, meaning we must get them in our diet because our bodies do not produce them. Branched chain amino acids have various functions related to energy production during and after exercise so they are needed in adequate amounts, but not excessive.
BCAA are often used to prevent fatigue and improve concentration. But the most relevant to you, perhaps, is the common practice of taking BCAAs to improve exercise performance and reduce muscle breakdown. While the first use has a fair amount of sceptics, the second is widely accepted in the medical world.