What does -12 mean you were looking at a fastpich bat and it had a -12?

The -12 is the 'drop' of the bat and means the number of ounces the bat weighs is 12 less than the number of inches the bat is in length.

If you have a 32 inch bat, a -12 drop means the bat weighs 20 ounces.

If you have a 30 inch bat, a -12 drop means the bat weighs 18 ounces.

If you know the length of the bat in inches and the drop, you can figure out how many ounces the bat weighs.