I am currently perusing through different scripts in the Creation Kit and am coming across the native keyword in the function declaration, with no code in the function itself.

An example would be:
Actor.psc - Function ShowBarterMenu() native

Is the code for this in another spot? where is this spot? am I able to change/view that code? or am I misunderstanding?

Apparently this means it is part of the source code?

Would that be something that is required by a skyrim script extender then, or is there no way to change how a native function works?

From the Creation Kit wiki:

The "Native" flag indicates a function that does not have a function body, because the function is implemented by the game itself. If you add the native flag to a function the game does not expose, the compiler won't complain, but the game will error at you.

So yes, it would seem native functions are those that are hard coded.