Informal sanctions are social responses that encourage or discourage behaviour outside a formal system of rules. Praise, a disapproving look or being left out of a conversation can communicate what a group expects.
They are often subtle because the expectation is taken for granted. Noticing them helps make unwritten norms visible. The same action may draw approval in one setting and disapproval in another, so the reaction tells us about the group and situation as well as the behaviour.