Well, this room was added in the "Claptastic Voyage" dlc. It's in the digital recreation of Overlook.
This door, which can be found on the left, part of the way up the road to the town square, has 3 lights above it, each representing a button you need to press.

The buttons have a timer that's fairly slow after a few minutes the buttons will reset. It should be possible to press all 3 solo but having a friend or 2 is recommended.

Anyway, the first button is on an outcropping behind Dave's shack past the town square

Butt-slam on that to start the timer and light the left-most light.

The second is on the distant rock formation past the valley of enemies where you fight the first form of Teh Earworm. It's behind the rock on the far side from Overlook

Hit this button to light up the middle light and run over to the Holy Spirits.

The third and final button is behind the Holy Spirits next to a loot chest on another rocky outcropping. It's below the Spirit so look over the edge.

Slam this button. The timer should stop and the door should be open.

Return to collect your Badass rank and... whatever else lies in that room.