By default, the Twitter app will automatically play videos nested in the new tweets as you scroll through your timeline. If you'd like to turn off this feature or use it exclusively when your smartphone is connected to WiFi (to reduce mobile data usage on your device), you can do so by changing a few of your settings.
Turn Off Video AutoPlay on the Twitter App
Open the Twitter app and tap on the Menu > Settings. Go to General settings > Video autoplay:
Make your selection from one of the available presets (Use mobile data and Wi-Fi, Use Wi-Fi Only, and finally Never play videos automatically):