A typical room type could be named “Single room” or “Double room”. The room type holds the room description, number of guests it can accommodate, price, etc. Your property may have multiple rooms (or units) that all are of the same room type. You can add and edit room types and units in Settings -> Accommodation.
A room type named “Double room”. No units or rooms have been added to it yet:
In this example the room type has two units added to it, see “Units of this type” to the right: