This means making the top of the screen (the main branding & nav area) distinct from the rest (the main content).

Why distinct top sections are good

The top section says “Here’s the top of the page”. Sounds obvious, but it feels good to know clearly where the page starts.

It also starts the site/page experience with a strong, bold statement. This is very “2.0″-spirited. We like strong, simple, bold attitude.

When & how to use a distinct top section

On any site, both the main branding and main navigation should be obvious, bold and clear.

So it’s a good idea to create a clear space at the top of a web site design that positions the logo and nav boldly.

Always put your logo right up the top of the screen. I’d always recommend putting your main navigation right after it.

It’s definitely a good thing to mark the top of the page with a section that marks out the high-level screen features as separate from the main site content.

The top section should be visually distinct from the rest of the page content. The strongest way to differentiate is to use a bold, solid block of different color or tone, but there are alternatives.

source: http://www.webdesignfromscratch.com/web-design/web-2.0-design-style-guide.php