It is the ratio of leucine:isoleucine:valine.
Of the three branch chained amino acids, leucine is the most important so that's why you see higher concentrations of leucine then the other two amino acids. Leucine is the amino acid in BCAA that directly signals protein synthesis through the mtor pathway.
It is a bit of a marketing scam to be honest. All 3 BCAA are very important, just leucine is the most important. I like the ratio of 2:1:1. Ratios higher then that (like 4:1:1) don't give you a high enough concentration of the other BCAAs which are still extremely important.