A reversing loop is a loop of track that makes a train turn around and go back in the opposite direction, as in the example below.
You cannot build reversing loops purely with LEGO® 9V Track because they don't work. Whichever way you switch the points, the track is shorting out the controller. If you follow the top rail from the start around the loop you will find that you end up at the bottom rail. It won't be just the reversing loop that doesn't work either, your whole layout will be dead if it has a reversing loop connected to it.
It may not always be obvious at first glance that you have built a reversing loop into your layout and you might wonder why it doesn't work. Look at #150 Series Layout E, this may not immediately look like a bad circuit but it is. The trick is to imagine a train running around the layout, if it is at all possible for the train to start in a particular place, run around the circuit, and then be going in the opposite direction when it gets back to the starting point, you have a reversing loop somewhere and it won't work.
Reversing loops only work for battery trains, so the above applies to layouts built with the 12V system as well.