What does "clue in" mean?

Also, clue up. Give someone guiding information, as in It's time someone clued us in on what's happening, or I hope they clue us up soon. This expression, which uses the verb clue in the sense of “inform,” is sometimes put simply as clue (as in I'll clue you—this isn't going to work). [Colloquial; mid-1900s] Also see not have a clue.