Meta event
A meta event is the rrweb event (type 4) that records context about the page rather than its content: the URL, and the viewport width and height at recording start. The replayer needs it to size the playback frame correctly before any DOM is rendered.
How it works
A meta event is emitted at the start of each recorded page and again on navigation within multi-page recordings, so a stream covering several page loads carries a meta event at each boundary. Players read it to set the iframe dimensions and display the current URL; analytics pipelines read it to attribute segments of a session to pages without reconstructing anything. It is a small event with outsized utility for indexing: URL plus timestamp per segment is often the first index a replay backend builds.
Where you encounter it
At the head of recordings, at page transitions, and in any "list sessions by page" feature, which is usually powered by meta events alone.