Source Licensing?

I was browsing the Valve Developer wiki a few days ago looking for new techniques in map making for my mod when I came across an article that talked about licensing source, and it mentioned a price. I tried emailing valve to ask them how much for licensing, but they haven't responded. I'm confused. I thought that valve let modders use source for free. What exactly is this?

Mods and the like are free and free to develop, but licensing is for when you want to sell your game. Games like Zeno Clash run on the source engine but they are licensed so the developer can sell it to the general populous just like any other commercial game.

Valve released the Half-Life 2 source code for free, but only the code that's included in the DLL's for HL2. That means that the only code they released was the code that covered front-end mechanics, logic, entities, etc. What is kept under wraps is the code to HL2.exe, or all the code that handles the graphics and rendering, BSP partitioning, shaders, geometry handling, and all that. Anyone who licenses the engine gets access to both the game code and rendering code. You'd only ever need engine/rendering source access if you were making some major deep-down changes to the engine.

Essentially, you're free to mod with any code they give you in the Source SDK.

Also, a license gives you the right to redistribute and sell the game with the engine, in whole or in part, with your modifications.

and the license has the source for a few of valve's games (including tf2)