Mirror
The mirror is rrweb's bidirectional mapping between the numeric node ids used in the event stream and the actual DOM nodes, maintained on both the recording and replay sides. It is what lets a compact event say "node 42 changed" instead of describing the node's position in the tree.
How it works
During serialization every node receives an id, and the mirror stores the id-to-node relationship. On the recording page, the mirror lets the recorder identify which serialized node a mutation touched; in the replayer, it lets an event find the rebuilt node to modify. The rrweb package documentation describes the public mirror accessors: record.mirror and replayer.getMirror(). Plugin authors and analysis tools use them to resolve events back to elements, for example turning a click event's target id into a CSS selector for analytics.
Where you encounter it
Mostly invisibly, but it surfaces when writing plugins, building custom analysis on the event stream, or debugging replay divergence, where an id resolving to the wrong node is the classic symptom.