Refers to a person who rents a room in someone else's house, typically sharing common areas. More specific than 借家人 (tenant of a whole dwelling).
He lives as a lodger in a house in Tokyo.
下宿人 is a boarder who may also receive meals, whereas 間借り人 typically only rents the room.
Compound of 間借り (まがり, 'room rental') + 人 (にん, 'person'). The reading まがり is a rendaku form of 間借り (まかり).