In short
A PLC is an industrial computer that controls machines and process equipment in real time based on signals from sensors.
PLCs withstand harsh shop-floor conditions and cyclically read inputs, run their program and update outputs. Motors, valves, conveyors and robot cells are usually controlled by PLCs.
On the software side, PLC data is passed to MES, SCADA and maintenance systems via protocols such as OPC UA, Modbus or MQTT.