I came across this essay by Kevin Mullaney. He goes over some basic conceptions about agreement in improv (what it is and what it isn't, and what it means). I think it's a helpful overview of what can be a tricky subject, particularly for newer improvisers.
http://kevinmullaney.com/2010/04/09/agreement-in-improv/
http://kevinmullaney.com/2010/04/09/agreement-in-improv/