It's a small rubber gasket that goes on the back of the oil pan between the pan and the block. You don't necessarily have to drop the oil pan to replace it. But if you're doing it right, you probably should.
Have you ever replaced your entire oil pan gasket, or rear main seal? If so, did you replace this gasket as well? It's part number 11121-22P00. Here's a link: NissanPartsDeal.com It shows up as 11121 on the diagram.
That page also has a good diagram of where it goes. There is a metal retaining piece (12296 in that diagram) that goes over it and holds a different gasket (12296E in that same diagram). You have to drop the transmission to get to it.
