What is radio firmware? When installing a custom ROM onto a rooted Android device, they often recommend to upgrade the radio firmware.

What exactly is Android's radio firmware?
The radio firmware controls basic low-level functions like network connectivity, Wi-Fi, and GPS. For several years, though, "radio" means "baseband", and controls network connectivity aspects only, while WiFi and GPS are governed by OS drivers, like any other devices.

Usually upgrading your radio will not give you any new features but will fix connectivity issues, increase range or performance, decrease battery usage, or the like. Some ROM's require you to have the latest radio versions to work.

While a botched radio upgrade can brick your phone, the process is basically the same as upgrading your ROM. Just make sure not to unplug or power down your phone in the middle, make sure you trust the person who released the radio firmware, and try to read some comments or reviews.