What is a linear function?

So today we are reviewing Linear Functions; and when asked what those are, I get a textbook-like explanation.

“It is where each x only corresponds to one y and graphs a straight line.”  Another student asks, “What does that really mean?  How does it even relate to what I would do when I’m done with school?”

I will post later on how my conversation went, but I’d like to hear your ideas before I give away any secrets.  (For reference, I teach 8th grade students in a public school on the reservation with high poverty rates.)