You're looking at the hotel from the side, with the front wall taken off. Guests walk in, queue at the desk, take the lift up, sleep, come back down and pay. Then the room's dirty.
Arrows move the yellow box. SPACE builds it or upgrades it.
The buttons in the top right open the guide book, the staff screen and the help. Or use the keys: G H ? X.
Do this first: build a room, then press H and hire a receptionist — without one, nobody can check in and everybody walks out. Then hire a cleaner, because a dirty room can't be sold.
Upgrade rooms for more stars. More stars means more guests and more money from each one, so an upgrade pays twice over.