much easier to show you a picture of one than to explain, but a classic saddle is essentially, two U's attached to one another, one upside down from the other and turned 90 degrees.

Saddles basically offer a lower pass through of a ridge, kinda like a mountain pass, and like us animals would prefer to take the easier path through/over a ridge which is often a saddle. If this is surrounded by woods, often times this pass can be almost completely isolated andout of sight. Sometimes you may not even know a saddle exists until checking out a topo.