What is a function in your own words?

A function is a black box, with a funnel to put numbers in, a chute where
numbers come out, a crank on the side, and some machinery inside.

You put a number in, turn the crank, and a number comes out ... usually
different from the one you put it.

If you do that enough times with enough different numbers, you can figure out
what machinery is inside.

Or, if you know what the machinery is, then when you put a number in,
you can always predict what number will come out.