The SerialDeviceDriver module creates a DeviceFS device for the serial port which is present on the 82C710-compatible I/O combo chip used in IOMD and Tungsten machines. It also contains a driver for the 6551 serial port used on early Archimedes machines, although that driver is not enabled in the RISC OS 5 build of the module.
The DeviceFS device which is created is compliant with the SerialDeviceSupport protocol, which allows the legacy OS_SerialOp SWI to be used to interact with the serial port.
Note that on Tungsten hardware the I/O combo chip actually provides two serial ports. The first port is made available via SerialDeviceDriver, while the second port is made available via Serial / DualSerial.