What does "sit in" mean?

Attend or take part as a visitor, as in My son's jazz group asked me to sit in tonight. It is often put as sit in on, as in They asked me to sit in on their poker game. [Mid-1800s]
Take part in a sit-in, that is, an organized protest in which seated participants refuse to move. For example, The students threatened to sit in unless the dean was reinstated. [c. 1940]
sit in on. Visit or observe, as in I'm sitting in on his class, but not for credit. [Early 1900s]
sit in for. Substitute for a regular member of a group, as in I'm just sitting in for Harold, who couldn't make it.