A ratchet is a general term for tools that use a tooth and cog system. Such as a ratchet wrench. Not really a slang word. It can also be used a verb, "to ratchet something", but I have never used it that way.

We don't use "ratchet" in a slang way down here, so I can't agree that there's a "common meaning". It's better to find out who actually uses the word as slang. Perhaps you could ask them, or even give us a sentence so we can try and help you understand it.

(U.S. native speaker) I've never heard it as slang. A ratchet is a mechanical device that allows something to be turned in only one direction. It makes a clicking sound. Colloquially, I have frequently heard the expression "to ratchet something up." For example, "they keep ratcheting up the amount of overtime we have to work." It means that something is raised but never lowered. It carries the idea that each new level becomes normal, and can't be reduced again--but can be raised. Each new level--each click of the ratchet--is called a "notch." "West is ready to ratchet up sanctions against Russia, says Hammond"--headline "11 Better Practices That Can Ratchet Up Performance" "Seattle teachers ratchet up movement against high-stakes tests"