A drop board deck is a deck that can be top mounted or dropped through. Usually if you have a drop board then it won't be drop through, but you can get a drop board that is also drop through. This makes the board closer to the ground and that makes it really stable.
Switchblade 36
store.thinkempire.com
The concave, rocker, and drop gives you a lot of grip when freeriding.
This is an example of a drop board/drop through.
Switch 37
stokedskateboards.com
This is an example of a drop board thats top mounted.