On the TYT MD-series radios, it's a simple timer. If the user has not keyed the mic in x number of minutes, the radio sounds an audio alert. If the user does not respond to the alert by keying the mic or pressing a button, the radio automatically broadcasts an emergency alert over the air with the ID of the radio. The feature is only available on digital channels because the radio sends the ID as a packet of digital data.
It's intended for commercial use, but can be handy if you're hiking or doing some other activity where a member of your group could be separated, lost, or injured. Or if you want your kids to check in regularly.