What does it mean when git says a file "needs update"?

Question

I can't for the life of me find any decent explanation of the "[file]: needs update" message that git sometimes spits out from time to time. Even the official git FAQ has explaining this marked as a TODO. If someone could explain A) what it means; and B) how to fix it, I would be extremely grateful.

Answer 1

It means you're trying to merge changes from somewhere, but the changes include modifications to a file that's dirty (currently modified in your working tree). You need to commit your outstanding changes, or stash them, pull/rebase/merge/whatever you're doing to update, and unstash