For the term, you can look at wikipedia : http://en.wikipedia.org/wiki/Test_stub
In a nutshell it’s a “fake” object that you can control that replaces a “real” object in your code.
For the function, what I understand is that and.stub() removes the effect of and.callThrough() on a spy.
When you call and.callThrough, the spy acts as a proxy, calling the real function, but passing through a spy object allowing you to add tests like expectation.
When you call and.stub, or if you never call and.callThrough, the spy won’t call the real function. It’s really usefull when you don’t want to test an object’s behavior, but be sure that it was called. Helping you to keep your test truly unitary.