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).