Stream key

Paired with the ingest URL, it tells your encoder, OBS, Streamlabs or a hardware unit, where to send video and which channel it belongs to. YouTube issues a persistent key you can reuse across streams and single-use keys for one-off events. Treat it exactly as you would a password. Showing it on screen while configuring software is a genuinely common accident, and whoever copies it can broadcast to your channel until you reset it. Resetting takes seconds and breaks nothing except the encoder profiles still holding the old value, so change it after any screen share where it might have been visible.