What do ''units'' mean in school education requirments?

The phrase "unit" can be different from one school to another. For a block schedule, it could mean one block of 9 weeks. In the more traditional type of school, though, a unit would the credit you get for one whole year in the same class. So, 1/2 unit would be the amount of credit you get for a half year (or semester) in the same course. Most schools require from 20 to 26 units or credits to graduate.