The browser will play the video even if the screen is off or when the browser is in the background. So you can listen your favourite music when your mobile is in your pocket with turned off screen. This will save your battery and protect against not-intended misclicks.
If you want to exit from the browser, use the exit button at the right top corner.

NOTE: the background playing function does not work on all devices. If the mobile has small resources (e.g. small amount of memory) the system can kill the browser when it is in the background.