D I/O stands for Digital Input/Output.
Digital Input (DI) is used to input binary-state (low or "zero" state and high or "one" state) signals by sensing switch closure or light beam breaks. For example, for positive logic, a zero-volt input is equivalent to an low input, and a five-volt input is equivalent to a high input.
Digital Output (DO) is used to output binary-state signals to actuate devices such as relays and solenoids.