Twitter is rolling out a new video (and GIF) autoplay feature today. This means ads, videos and Vines will play automatically as you scroll through your timeline. Annoyingly, it's enabled by default, but you can disable it -- here's how.
If you access Twitter via a desktop browser, click on your profile icon and select Settings. Under Account, you'll see an option for Video Tweets. Make sure Video autoplay isn't ticked.
On the iOS mobile app, head into Settings > Video autoplay and tap on "Never play videos automatically". (The autoplay option isn't yet on Android but Twitter says it will appear in a future updates.
These settings are rolling out slowly, so if you don't have them yet, check back a bit later.