A test bank is the set of questions and/or possible tests that can be given in a testing period. The idea is that you are able to make it harder to cheat on a (usually multiple choice) test by making potential cheaters either not know or not have access to others answers on a test.

Basically, the professor was too lazy to come up with new questions each year, so he just reused old questions. Even worse, the professor didn't come up with the questions himself; he used the questions provided by the publisher of the textbook he was using.

After all, it's not like the publishers don't also sell study guides which include many of those questions...