Tree shaking is essentially reducing the amount of included code to only the stuff being used.
Let’s say you have a JAR, DLL, or whatever with 5 functions.
Your application calls function A which also calls function B of the DLL/jar.
Tree shaking says “hmm, if I follow the code execution path, I only need 2/5 functions from the DLL/jar” and discards the remaining 3/5 functions.
This significantly reduces the bundle size (the bundle is what is sent to the browser).
It’s maybe a poor example, but it is what the plaintiff is alleging, so I think it is a good analogy