Behind the inside of the dugout are tunnels leading to the "guts" of a stadium. Ground crew equipment, practice equipment, exits to the outside, etc. But most popularly, the clubhouses and training rooms are down there.

Stash of Playboy and Hustler

A washroom right beside the dugout and the clubhouse and everything associate with it behind the dugout. That includes an indoor batting cage.

the locker rooms

It depends on the specific stadium - there are different layouts. In general, though, the team's clubhouse is back there, "clubhouse" meaning "locker room, showers, exercise room, manager's office, and a few other things."

A row of very expensive seats. On top of that, the dumbass usher morons tell you to take your stuff off of it. You can't even put your feet up, for crissakes!
There are also a lot of peanut shells, spilled drink stains, and other stuff I can't even identify.
Everyone else seems to think that you asked what was inside the doors leading out into the dugout. You asked what was behind it, and I told you. Enjoy! . . . .