Amino acids are the basic building blocks of all proteins. In total there are 20 amino acids, which are all required for the producing of the body's own proteins. Particularly important amino acids are the essential amino acids which the body cannot build itself, but they should be taken from the nutrition or supplements. 3 of 8 essential amino acids are called BCAA; which stands for "Branched Chain Amino Acids". During the training the large quantities of amino acids are naturally released from the muscles and used to produce energy. If you take essential amino acids in sufficient quantity, the body has less recourse to body muscles.
Products with essential amino acids, such as BCAA, should ideally be taken before or during the training.