They’re a mesh walkie-talkie, but you don’t need to walkie or talkie 😁

Meshnet means that if A can see B and B can see C, then A can message C, it’s routed through B automatically.

Also it’s text only, not enough bandwidth for speech

walkie talkie

Yeah, I think so. I saw a video where someone called it "a walkie talkie, but for sending text messages". People use these for going on remote hikes, hunting trips, or protests. Basically, any area where you can't use a cellphone. They're not a replacement for cellphones, they fill a different usecase.

I think Ham radio means hobby and amateur radio, I.e. not professional. Radio is a type of radiation at very long wavelength. From the wiki:

long waves can diffractaround obstacles like mountains and follow the contour of the Earth (ground waves), shorter waves can reflect off the ionosphere and return to Earth beyond the horizon (skywaves), while much shorter wavelengths bend or diffract very little and travel on a line of sight, so their propagation distances are limited to the visual horizon.

Others have explained mesh pretty well.