What is letterboxing?

I meant to put a link in the last post about letterboxing.

But the boys were playing with playdough and telling stories and then both of them asked at about the same time, "Mom, are you listening to me?"

To which I honestly replied, "no, I am not."

Then JD told me, "you know, Mom, children are more important than a computer."

And he's absolutely right (though I do think a little bit of time on the computer is good for me, I can let it consume too much time) so I closed up my laptop and forgot about giving you a link.

So, here it is now:
What is letterboxing?