What is a teaching prep in terms of teaching load?
A "prep" is an individual different course you are teaching.  If you are teaching 2 different sections of the same course the same semester, that counts as only 1 "prep" but counts twice towards your "teaching load".

Given not-too-large class sizes, teaching 4 courses with only 2 preps might be less work than teaching 3 courses with 3 preps, since presumably you don't have to prepare twice to teach the same class twice.