BCAA's are branched chain amino acids, namely, Leucine, Isoleucine and Valine.
They are all essential amino acids.
The point of taking them before training as opposed to just getting some as part of a full spectrum of amino acids in a protein source like whey or meat etc, is if the body has to use the ones in muscle then this stops protein synthesis.
By taking them at quite high dosages this usage from muscle tissue is prevented and no cessation of protein synthesis occurs.
Mauro Di Pasquale and Charles Poliquin were suggesting pretty high doses before working out which can be pricy, up to 40-45 grammes for some people. Very basically you don't get the same effect just consuming food with them in, in this case.