You select a theme of your choice, and can look at the dungeon and compare it to the mirrored empire dungeon. In the upper right is a spot for taking a close look at your workers, and the upper left has a minion for you to possess if you want to get a REALLY close look. There's also a series of maps duplicated like this but with different themes on the empire's dungeon, so you can compare all the themes.

and how can i use it? sry if i´m dumb :/

It's for trying on different dungeon themes. It's a dressing room.

i don´t understand