If you call someone a know-it-all, you mean they act as if they already know everything, and they like to correct other people. You say it because you do not like that behaviour — it is never praise. The person may really know a lot, or they may only sound sure; the word is about the show, not a test score. It is informal and a little sharp. You hear it about a classmate who cannot let a small mistake pass, or a friend who explains your own story back to you. British English often says know-all for the same kind of person; American English likes know-it-all. You can put it in front of a noun too: a know-it-all tone, a know-it-all smile.