"'Yes, and' is about agreement. Agreement doesn't necessarily mean you literally say the word yes all the time... In fact you more often probably do say no. Agreement means I'm agreeing with the improviser, not that I'm agreeing with the character... Agreement is I'm agreeing to your premise."
- Matt Besser on the latest episode of improv4humans explaining the common misconception over what "yes, and" means. It's part of a well-articulated rant on the subject that shows why he is an improv legend. You can download the episode on iTunes