Answer:
Et al. is short for et alii which is Latin for "and others". It is used in place of usually rather long lists of names who contributed to a work and one runs out of room. Usually it is used for multiple authored works such as encyclopedias, dictionaries, or other papers etc. "Terry M. Banks, Joseph H. Dunkin, Thurston J. Collins, et al." (meaning more authors are involved but these isn't enough room to state them all).
How I usually use it is when there are more than three authors and at least two are not mentioned (hence the plural). If three authors, all three should be named. If four authors, two are named and two are et al. If five or more, three are named and two or more are et al. However, the Chicago Manual of Style has other ways for it to be used.
I recently used it thus: Mr. Hitler, et al.