Dorm rooms are rooms that are shared with other people. The number of other people staying in the room depends on the hostel and will be mentioned in the room description. They will usually have bunk style beds. In a dorm room, you need to book a bed for each person in your group. You can't share a bed in a dorm room with another person.
There may be lockers or safety deposit boxes where you can store your valuables in the room also.