What is a fixed header?
A fixed header means that as the page is scrolled down past the header, the header logo area and menu stay displayed at the top of the page, maintaining the background color you set for the header. There are several types of fixed headers, and they vary a bit by the layout you choose, as described here.
Are fixed headers good?
A fixed header can give a website a polished look and has the added benefit of saving users time (since they don't have to scroll up to get back to the main navigation menu). In fact, one study found websites with sticky headers are 22% faster to navigate than other websites.