A PLC (Programmable Logic Controller) is similar to a microcontroller development board, like the Arduino Uno, except beefed up and compliant with industrial standards. They’re most often used in industrial automation, and the Controllino is designed to bridge the gap between microcontrollers and PLCs with Arduino-compatibility. In this case, GreatScott! is using the Controllino PLC to monitor a light barrier in his garage and then set off a strobe light and siren if the barrier is broken.
Light barriers are simple and common, and you likely already have one if you have an automatic garage door opener. An infrared light beam is sent from an emitter to a receiver across the entryway. That makes it easy to detect if anything interrupts that beam. Because the Controllino is Arduino-compatible, GreatScott! was able to program it using the Arduino IDE. For now, he has it setup to activate the strobe light and siren if someone crosses the light barrier behind his garage door. But the Controllino has many versatile inputs and outputs, so GreatScott! can easily expand his garage security system in the future.