The Dash.net service is a combination of hardware and software, hooking up your in-car navigation system that connects to the online dash.net using WiFi or the cellular network. The device has a GPS system for normal GPS-navigation functions, but also can connect to other Dash.net users/devices while on the road to create a mesh of Dash users.
What does the mesh do? Well, it will "supply traffic information, destinations, nearby service locations, and weather as well as 'all the kind of stuff you could imagine,' said Lego", the CEO of Dash. If you're stuck in a traffic jam, your device will relay that information to the central Dash servers, which in turn will get propagated to anyone who's in the vicinity, prompting them to avoid your mistakes. Sounds like GPS done right.