Keith and Royce - when you create a container for your content on a web page - either with a table or with a "div" - you set the properties of it, width, height, cell spacing, cell padding, backgrounds, borders, and a few other variables.
So if I create a table that's 800 pixels wide I only have that much room for the content. If I use say 125 for the navbar then I only have left 675 pixels, so any other picture or other content has to fit that dimension or the page won't display properly. The width will lnever go wider than the 800 Pix setting.
Now instead I set that width property for 80% say - it depends on the monitor settings how wide it can, will and does go. More of a dynamic dimension, sort of. I can put a picture there of 800 pix and it will add the 800 to the 125 for the navbar, plus any spacing between, and stretch to what it needs. I believe I'm over simplifying but this is the basic premise.
Hope this helps