On Twitter, users share a lot of automatically triggered GIFs and videos; automatic triggering is not a bad thing in principle, as it makes you jump to the content faster, but if there is a lot of animation, it can be distracting.
Twitter has therefore made it possible to disable autoplay. The option can be found under Settings | Privacy | Utilities | Autoplay.
If you don’t want videos to start automatically, this option should be set to Never.