I can see why you are confused.
Quite simply, the get() call you see above and the get() function described in the reference are two totally different functions.

A plain old call to get() returns information about the pixels currently drawn on the screen - either at a point or as an array of colors.

But what you're asking about is not a plain old call to get(). It's a member function of a object, possibly an AudioPlayer object. The "in.right." before the call to get() says "you know that input we have? You know the right channel of that input? Well I want to get() information about the right channel of that input."

I don't know what getLineIn() does, so I can't compare the two.

Interestingly enough, a plain old call to get() is also not really a plain old call to get(). It's a call to the PGraphics object g.get(), which is why it gets pixel information.

To know exactly what the call to  in.right.get() is doing, you'd have to look at the reference for the member functions of the object type that your variable "in" is!

Does that help explain it?