每间 is a fixed combination of the determiner 每 (every) and the classifier 间 (used for rooms). It functions as a determiner or pronoun, referring to each individual room in a set — you'll often see it in descriptions of hotels, apartments, or buildings. For example, 每间房间 means 'every room' as a noun phrase, or 每间 can stand alone as a pronoun meaning 'each one (of the rooms)'.