Gmail’s Web Clips mixes news headlines with contextual ads. Gmail has recently started showing ads in the Web Clips box, if for some reasons you don’t like to show Web Clips box, you can disable it from the settings page.
Gmail’s help center article explains that “Web Clips show you news headlines, blog posts, RSS and Atom feeds, and relevant sponsored links, right at the top of your inbox. Each clip displays the source from which it was received, how long ago the clip was published, and a link to access the entire story or page containing the clip. From your inbox, you can scroll through clips you’ve already seen by clicking the left arrow or see new clips by clicking the right arrow.”
Here’s the direct link to disable Web Clips.