A 406 MHz beacon is a device, usually an Emergency Position Indicating Radio Beacon (EPIRB), an ELT, or a PLB (Personal Locator Beacon), that transmit distress signals on the dedicated search and rescue frequency 406 MHz. They utilize the COSPAS-SARSAT and GEOSAR satellite systems by sending a distress signal on 406 MHz frequency that is used to pinpoint your exact location. The coordinates are then sent to the appropriate agency to begin rescue efforts.
Further reading about EPIRBS and Personal Locator Beacons (PLBs):
How to Register an EPIRB or PLB
Marine Electronics Review - SPOT Personal Locator Beacon
Marine Electronics Review - ACR Electronics AquaFix 406 I/O Personal Locator Beacon
Marine Electronics Review - ACR Electronics ResQFix 406 GPS Personal Locator Beacon
Marine Electronics Review - ACR Electronics GlobalFix iPRO EPIRB
