In a traditional network, devices all connect through a central point, and if that point fails or is out of range, they are cut off. In a mesh network, each device can relay traffic for its neighbours, so data hops from one to the next and can find another path if one link drops.
This makes coverage easy to extend, just add more nodes, and makes the network resilient, since no single device is essential. The cost is complexity in deciding routes and some overhead as devices coordinate, but the robustness is valuable where a single hub would be a weak point.