Meshtastic is radio powered mesh network, based on LoRA technology, operating license free in the ISM band, in the US anyway. Other countries use different frequencies I can’t comment on.

The idea is that you can send text messages to each other, either in a small group of friends, say camping or hiking. Or to a large group of people, perhaps even city to city if the mesh is strong enough.

All of this works using off the shelf radios, flashed with custom firmware. No other infrastructure or Internet is used (though you can involve the Internet if you want, it’s not required and many people don’t like it)

One radio talks to another, which talks to another, and repeats your message down the line of radios that are near each other.

That way, even if you couldn’t reach your friend directly point to point, the message will get bounced to him.

You don’t have to own the repeating radios in between, and those radios can’t read your messages because they are encrypted. They only forward them.

The strength of this encryption is up for debate, at least the last time I checked. It likely wouldn’t stand up to intense scrutiny or attack, but is more than enough to keep casual conversation confidential.

All of this can be yours for the starting price of $20-30 for a radio. You use your phone to compose and read the messages, your phone connects to the radio via Bluetooth.