If the browser supports the Request Video Frame API then this property will hold the metadata that is returned from the callback each time it is invoked.
See https://wicg.github.io/video-rvfc/#video-frame-metadata-callback
for a complete list of all properties that will be in this object.
Likely of most interest is the mediaTime property:
The media presentation timestamp (PTS) in seconds of the frame presented (e.g. its timestamp on the video.currentTime timeline). MAY have a zero value for live-streams or WebRTC applications.
If the browser doesn't support the API then this property will be undefined.