i8042prt.sys is a driver for the ps/2 ports commonly found on pc's.
The ps/2 ports are controlled by a chip that was made by Intel, the Intel 8042. It is some kind of a bus, that uses one port to talk to the controller and another one to talk to the devices. The devices have separate interrupts so they can notify the system when they have something to say. When that interrupt is received though, communication happens through the same port for both devices.
This means that there has to be a driver that coordinates access to these devices, or there will be problems when for example you move your mouse while you press a key that turns on one of the keyboard leds.
The Windows i8042prt driver is pretty much documented on MSDN. Most of the documented stuff should work in the ReactOS driver too, so hopefully third-party drivers, like for touchpads and other weird mice, can be made to work.