1. An intermediary computer that does set-up and filtering for another (usually more powerful but less friendly) machine (a back end).
2. What you're talking to when you have a conversation with someone
who is making replies without paying attention. Look at the dancing
elephants!
Uh-huh.
Do you know what I just
said?
Sorry, you were talking to the front end.
3. Software that provides an interface to another program ‘behind’ it, which may not be as user-friendly. Probably from analogy with hardware front-ends (see sense 1) that interfaced with mainframes.