If you're referring to the section of the timeline you've highlighted in yellow, that's the Select Range tool which is normally used to select and then produce only the highlighted section of a project. You can also choose to render a preview of just that section if it contains a lot of CPU-intensive edits that prevent smooth playback.
See the online Help file
here (you may need to scroll up a few lines to see the full section) and
here for more info.