https://meshtastic.org/

In short--it uses radio waves (similar to HAM) to send text messages.

It does not require any connection to the internet to do so. Messages can be encrypted if you choose.

It requires a small LoRa radio device ($25-50 or so) to establish these connections.

You can use a phone app to connect to the LoRa if you choose to help configure it, but it's optional--there are self-contained #meshtastic devices with screen, keyboard, etc. that do not require any other equipment.

Also--the meshtastic devices establish their own "mesh" network with other devices in range, to extend the network automatically.

These are the basics (although there's quite a bit more to it if you want to dive in deeper).

Best use case is communication when the internet is down, or not available (e.g., if you're hiking in the wilderness). Or if you just want a secure text service that's not subject to any existing infrastructure (e.g., wireless networks, cell service, etc.)

Happy to answer any additional questions.