Mobile VPNs work just like ones for desktops and laptops, but on your smartphone. The app on your phone routes traffic via a VPN server you connected to, as well as encrypting outgoing data and decrypting the incoming.
This means that if data is intercepted between your phone and the VPN server (say, by a sneaky hacked Wi-Fi hotspot), it’s unreadable to the thief. Meanwhile, the routing replaces your phone’s IP address with the VPN servers, making all the world think you’re connecting from the server’s location. This tricks websites into thinking you’re in that country and keeps your real location secret.