SSI is an application which enables a PC computer to control the operation of a model railway layout through hardware connected to the communications port.
SSI uses the MRAPI 'middle ware' product to provide the actual hardware communications.
SSI replaces a conventional control panel with a computer screen, providing significant levels of functionality that would otherwise require complex wiring. It does not 'drive' trains.
SSI uses the 'signalman centric' (signalman tells trains what to do, driver drives according to signals) rather than 'driver centric' (driver drives trains and system changes signals depending on where he is) paradigm.