Amino Acids are the parts that are combined to make proteins. Some are essential and others can be made within the body from other amino acids we eat. Branch Chain amino acids (BCAA) are the 3 essential amino acids most consumed during exercise to form glutamine used in the liberation and generation of muscle energy.
Generally suplementing with Amino Acids is done to improve the protein content of 'lower grade' protein sources. Supplementing with BCAA prior to, or immediately after, training helsp to reduce muscle tissue breakdown.