Behind The Rule: GMs Shouldn't Say No

We have a lot of unwritten rules in Table Top RPG land. Each group has their own collection, and various subgroups and niches have variations on them. In the land of GM advice, one of the most commonly bandied about tropes is that as the GM you should never say no to your PCs. You should say "yes, if...," "yes, and...," or "yes, but..." but you shouldn't say no. But what does that really mean? Can you really not say no? Today I want to look at this piece of advice a bit closer, break it down, and suss out the core wisdom it is trying to impart.