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That's not a stupid question. It's a bit of a tough topic. I gotta zoom out a bit to give you an accurate answer but I promise I will :-)
Wizards published D&D 3.0/3.5 under a license called the OGL. It says that OGL books can reference other OGL books and to do so you need to publish under the OGL.
Pathfinder was published under the OGL. You can reference Pathfinder (and any other OGL book).
The Pathfinder Compatibility License (PCL) is an extra thing. It lets you put that Pathfinder Compatibility Logo on the cover. You need to sign up for it (it’s free). For product to have the PCL logo on it it needs to be, you guessed it, totally compatible with the Pathfinder RPG. This means it can’t be added to like a standalone game (though it can be compatible with other systems as well) or something that doesn't rely on Pathfinder to work.
The PCL doesn’t mean Paizo is affiliated with you or endorses your product. There are a few other conditions (you gotta send a copy to Paizo; which you’ll do anyway to get it on their store, you gotta reference stuff in a certain way, etc) but BASICALLY it lets you put a badge that assures people that your product works with Pathfinder. It’s not an endorsement, but it looks very spiffy and helps folks identify Pathfinder products from other publishers on the shelf.
And all of this is SUPER cut down; there is way more to it than that. Here are some useful links:
OGL: http://www.opengamingfoundation.org/ogl.html
PCL: http://paizo.com/pathfinder/rpg/compatibility