It's not about the "cut" you're thinking; it refer to in-app purchases.

Once you bought a game, Valve keep demand a 30% cuts on anything you sell once the customer launch your executable (.exe, binary file/game engine).

hypothetical scenario to help visualize (it won't go like that most of the time, but useful to understand the concept):

• customer Install and Launch Steam
• customer buy (Valve earn 30% cutshare) and install game on Steam
• customer uninstall Steam, keep installed game
• customer launch game (if is made in a way don't need Steam dependencies).
• Anything sold while game engine is running must give 30%,of further earning, to Valve.