The Pit is, well, a giant black pit off to the side of Hueco Mundo. Jumping into the pit will send you to Las Noches, a place that can be used for farming Hollows and Menos, and contains a few Mission Boards.
To get to Hueco Mundo you need to use the Tunnel in Karakura town. Check the above image to see the entrance of the Tunnel. This tunnel is different from the one you use to become Quincy because it has all-white tiles. While the Quincy Tunnel had Blue tiles on the right and white tiles on the left.