The "slot" is basically when your right elbow feels like it's almost hugging your hip. Many players "drop into" the slot after taking a backswing with a loose elbow, and many others keep it in the slot the whole time. I'll show you an example of each.
This is Jim Furyk. He's an extreme example but he's a good one to demonstrate someone who doesn't take it back on the same plane as he brings it through, but still gets his elbow in the right place.
Look at the bottom row, 4th image, and notice how high-and-away his right elbow is from his hip. Now look at the next image to the right and look where his right elbow is: locked right in on that right hip. That's what keeps him from slicing the ball.
Now look at Ernie Els. Notice in the third and the fifth picture that his elbow is in virtually the same place both on the takeaway and on the downswing. It never leaves the slot so it never has to get back to it.
I prefer the latter approach because I think it's easier to get it on exactly the right track, especially for someone who doesn't play enough to get the "feel" right for where the slot is. But either way, try to have someone videotape you from this same down-the-line angle and watch your elbow. It tells huge stories about where your ball is going before it even gets airborne.