Live caption is a feature that is exclusive to the Pixel 4 at this moment.
With Live Captions turned on, hearing impaired users can now enjoy English langauge videos clips with English captions even it wasn’t subtitled.
The caption is real time and is very useful for LIVE broadcasts which are usually come without subtitles.
Live captions can be enabled in set Settings -> Accessibility options. It is currently limited to English language only.