Regular SSH won't work if your IP address changes since it uses TCP where sessions are tied to (IP address, port) tuples. However mosh uses UDP and its own session management scheme, so you can "roam" between IP addresses and your session will stay alive, you can continue typing and will receive screen updates as if nothing has happened.

"roaming" as a technical term just refers to any case where you move between networks and something maintains a connection. mobile phone network roaming is just one case of that, but e.g. moving to a different WLAN access point is the same. In the case of mosh, you can change your IP but it will still continue the existing session with the server (unlike SSH, which will fail the connection)

Normal SSH connections drop when you switch WiFi networks.

With mosh helps you keep your connection alive when roaming, e.g you start a connection on a cafè Wi-Fi while having breakfast, connect to your VM while tethering your phone, go home and switch to your home wifi