What does this actually mean. I know the library talks about millis and returning it. I dont know what this means. What does it mean to "return"?
Think of "millis" as a mathematical function, you call "millis", and it gives you back (returns) a value. The type of that value is unsigned long, so a 32 bit unsigned number.
"Calling" a function means that a new workspace is created for it, the argument values are placed in it, and the code the function is run.. When the function code is done its result value is in a known place in its workspace, and returns passes back to the calling code (the place to return to has been remembered), which then uses the returned value as it wants.
The place where all this workspace exists is a Last-In-First-Out stack in memory. On some chips the arguments and or result can be passed in known registers rather than on the stack.
The key elements are a mechanism to pass arguments and result(s), a stack to allocate temporary workspace for running functions and remember return points.