Screen real estate is the usable area of your screen. In the case of browsers this is the area the page displays in excluding all toolbars, statusbar, titlebar and frames. Ever notice that the browser toolbars take up a lot of space? If you are using Internet Explorer you can reduce the space they take up quite dramatically. This gives you more room to actually see the page your interested in. I have copied the toolbar layouts I use for both IE and NN browsers for your reference below. Notice how IE really lets you minimize the toolbars while still giving you access to all options. IE uses 50% less space to display the same options. Key IE advantage is to be able to move groupings in whatever order you want. Overlaying as desired. A very flexible toolbar.
The next versions are for when you really want to shrink things down. Notice that I have to hide options totally in Netscape? In IE I still show the key buttons and the address field. I could have even put the buttons before "File". Clicking the ">>" shows all the other buttons that are hidden.
So as you can see how you setup your toolbars can have an effect on your viewing pleasure. I don't know about you but I want to see the page not all the stuff around the edges. Any way I can expand my viewing space(screen realestate) while still keeping functionality I will. Once you get used to it, you will wonder why you ever took up all that space for toolbars.