Welcome to FT, mlw.

The quick version:
Each letter represents a fare class. P and F are first class fares, Y is full-fare coach, B and M are near-full-fare coach, other fares are cheaper.

The numeral to the right of each letter shows how many tickets are for sale from that "fare bucket". However, there are a few tricks, for instance:

P and F are not independent of each other, so P2 F2 does not mean that there are four first-class seats open on the flight--there may only be two.

9 is the maximum number that's displayed. If a value for any fare class is less than 9, it's 'real'; a 9 simply means there are at least 9 of those tickets available.