You have the basic concept right, plus they also share GPS coords for the units for tracking to other users/nodes.
They're neat for what they are, but their throughput is very small. So you can send simple text messages and coords, but not much else. You can't do voice or large data transfer over them. But they can be setup to sip power so they operate on battery for a while, and the mesh nature means you can expand range across the network by adding more well spaced nodes.