Beacon mode, a feature of many electronic keyers, is the continuous repetition of a message until the operator intervenes.
Any ham can operate a beacon, or continuous, unattended transmission, as long as the transmission itself is in accordance with specific rules. An electronic keyer can be set to send a repeating message, and there are some that will resume sending after the power supply has been interrupted.
A more common use of beacon mode is to send a “beacon CQ.” You program the keyer to send a CQ call, wait for a few seconds (during which you are listening for a reply), and then repeat the CQ call. Typically the beacon mode will be terminated as soon as you touch the paddle, which you would do as soon as you hear someone replying.
A “beacon CQ” capability is very useful during contests, and it can also be helpful when the bands are not particularly active and it’s necessary to call CQ many times before getting a response.
If you use a “beacon CQ” you have a responsibility to actually listen between the repeats, and stop the repetition as soon as you hear another station. It’s pretty embarrassing to take a break and come back to the radio to find that you’ve been calling CQ the whole time you were away!