The counter performs a simple, but interesting task: it shows how many people are currently watching a video. Google is testing the feature in its app for Android. Wrote about it on Androidpolice.
A string with the number of spectators located between the video title and buttons like/dislike. It’s small, so won’t distract from viewing.
Today, this feature is available only to a few viewers, but if the test is successful, then soon it will appear to all users of the service.