Fixed width is when you fix your sites width to a certain value, for example 800 pixels or 80% of screen. Right now, I don't know if it's only with widescreens or with all, but I feel like I need to look all over the screen at once which is very confusing.

Also like Rob already said you have too much content on side bars. When you scroll down you have this big, blue box with nothing inside because sidebar is too long. Better add some of this content to your.. well, content instead of sidebar.