It means that you can only bid by 20s.
So, for example, you could bid up to 20, 40, 60, etc dollars.
You wouldn't be able to bid from 20 to, say 30.
You are able to bid from 20 to 40, or 20 to 60 - your gap in between doesn't matter, so long as it's by 20.
Understand, Barn?
