Parameter "this" keyword : GetBottomCenter(this Rectangle rect)

I'm pretty new to C# and I noticed in the xna 3.0 platformer starter project the have the method: public static Vector2 GetBottomCenter(this Rectangle rect) { return new Vector2(rect.X + rect.Width / 2.0f, rect.Bottom); } I'm just wondering what declaring a parameter with "this" does, I looked up the keyword on msdn and it only shows the uses I know of, like accessing member variables using this., and when I google it, I just get a bunch of sites about different ways to pass parameter that don't mention using "this".