YouTube rolled out a big update earlier this year that focused on tabs across the top of the screen. For most people there were three, but a fourth one showed up sometimes too. Now that fourth "trending" tab is official. In a post on the YouTube blog, we're told that the "new" trending tab will list all the hottest things posted to YouTube on a daily basis.
The Android app has the tab of course, but there's a trending section on the web too. They appear to show the same content, but the Android app has quick links at the top for different sections like music and games. You can also check out the YouTube 2015 rewind content, which is a collection of the biggest stuff on the site in the last year.
This should be live in everyone's app at this point as it was posted to the global blog.
- YouTube Blog