空室の表示が出ている。
The vacancy sign is on.
満室 means all rooms are occupied, while 空室 means there is a vacant room available.
空地 refers to vacant outdoor land, while 空室 refers to an unoccupied room or apartment, so the two should not be confused despite both starting with 空.
空室 refers to an empty room, such as in a hotel or apartment, while 空席 refers to an empty seat or vacant post, so the two are easily confused by shape but differ in referent.
Compound of 空 (empty) and 室 (room).