It’s… really not complicated. Look, take a bowl, okay?
Sit the bowl on the table in front of you like you’re going to eat out of it. This is a cavity, so it’s a mold. You’d end up with, uh, another bowl.
Now, turn the bowl over. It’s upside down and it’s a projection, so it’s now a plug. You’d end up with, uh, a big half-sphere.
Get it?
If you still don’t… Google (and reading) is your friend.