A cloister (or monastery) is a quiet little stone building, surrounded by a 3×3 patch of countryside. A folk placed on one is a monk.
When a cloister scores
A cloister scores 1 point for itself plus 1 for each of the eight surrounding tiles. The moment its eighth neighbor is placed, score it: that's the maximum, 9 points. The monk goes home.
Incomplete cloisters at end of game
If a cloister hasn't filled its 3×3 ring by the time the bag is empty, score the building plus whichever neighbors did show up. Anywhere from 1 to 8 points. The End-game wizard will prompt you to enter how many tiles surround it.
A friendly tip
Cloisters are reliable. You can drop a monk and walk away knowing you'll likely get most of those nine points, even if the ring doesn't quite fill in. Good first place for a new player's folk.