For me, some of the things I felt out of the loop on at university, for various reasons:
Knowing what people meant when they said things like ‘left wing’, ‘right wing’, ‘daily mail reader’, ‘guardian reader’, ‘radio 4 listener’ ‘Texas republican’ and the stereotypes associated with these people.
Knowing what a crouton is in soup, what a fish knife is. Basic foreign dishes (pad Thai etc). White wine with fish, red with beef (and confidence to break the rules or to ask what a particular menu item is). Knowing how to order and pay in a group situation in a restaurant.
Basic high level knowledge of history that is taught in UK: Russian revolution, US civil war, British empire, Israel/Palestine, India/Pakistan, who Stalin, Lenin, Franco, Mussolini were.
Knowledge of TV/movies and famous scenes or quotes, like Star Wars, Monty Python. Why people say ‘I don’t belieeeeeeeeve it’.
Watch popular TV, movies based on true lives/historical eras, satire (bbc and radio 4), read the news and the comments.
Read widely and make an effort to use a wide range of vocabulary in every day life.