front end (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!

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.