Facebook videos are huge. Part of the reason why Facebook videos have been successful i.e. one gets lots of views is because they autoplay. On desktop, on the mobile app, they autoplay by default. Facebook doesn’t make things any easier as the option to change that is buried in its horrendous settings section.
There’s no easy way to do that on the mobile application. You just have to hit the humberger menu then scroll down to App Settings in the Help & Settings section. From there you get this:
Choose ‘Never Autoplay Videos’ and you’re good to go.
On desktop or mobile web, it’s easy, just go to https://www.facebook.com/settings?tab=videos and you’re done.