The #main divs width is based off of the browser window width by giving it a 20% value for its' left and right margin, instead of a fixed width in pixels. So, it will always have 20% of the window space on both sides of it.
The #fixed element is given a width value in pixels(non flexible width), but it is given a % value for its' left margin that will match up +/- with the content areas leftover space of 20% when a user resizes the browser window.
100% window - 20% #main right margin = an 80% left margin on the #fixed element to make it line up with the right edge of your content area: even while resizing the window(until it gets to small). Notice that if you tried giving the #fixed element right: 15%; instead of a left % value, it will appear to move relative to the edge of #main as you resize your browser window, and not line up.