Adjoining rooms are hotel rooms that are connected to each other and able to be booked together to make one larger entity.
Most of the time, they are also adjacent rooms, with a door or entryway connecting the two; however, they may be adjoining rooms without being adjacent rooms, perhaps through the utilization of a hallway, stairs, or some other passage.
Some places define adjoining rooms to have a connecting wall without a doorway, but usually, adjoining rooms are connected by a door that locks.
Often, there is a door on both sides, opening inward to each room and able to be locked on either end, so that privacy can be ensured in either room. Having doors that lock on each side allows the hotel to use the rooms separately for two different sets of guests.
For more, see this post: Adjacent Rooms vs Adjoining Rooms.