Discrete is a signal sent either to the PLC (input) or from the PLC (output). The signal has two states (either ON or OFF). A Discrete I/O (I/O stands for Input/Output) device or module recognizes only ON/OFF signals.
Discrete is nothing but digital. It is the most common type of I/O module. This is a type of interface with input devices such as pushbuttons, selector switches, and limit or proximity switches.
These switches are simply either on or off. The output control is also on or off and usually consists of lights, relays, solenoids, and motor starters.