Full Speed is one of the original USB speed tiers, running at 12 Mbit/s. It dates back to USB 1.1 and was, at the time, faster than the very first Low Speed tier used mainly for basic input devices like keyboards and mice.
By modern standards, 12 Mbit/s is extremely slow, far below even the 480 Mbit/s of High Speed USB 2.0, let alone the multiple gigabits per second of SuperSpeed. Full Speed is still used today for some low bandwidth devices, such as certain audio interfaces, sensors and microcontrollers, where the higher speeds of newer USB generations are unnecessary.
Seeing a device described as Full Speed does not mean it is faulty or outdated for its purpose, since many devices simply never needed more bandwidth than this tier provides. See our guide to USB speeds for how this tier fits alongside the faster ones.