n. 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!"
"Do you know what I just said?"
"Sorry, you were talking to the front end."
See also fepped out.
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.