The NAND module is a logic gate with two inputs. Think of it as “not and.” The NAND module will always send an on signal unless both input one and input two are receiving an on signal. It’s the exact opposite of the AND module.
Real World Example
Think of a car with two doors. When the doors are closed, they activate inputs. Inside the car, a light shines if the either or both of the doors are open. The light goes off when both doors are closed.
The first lunar landing module computer was made up entirely of thousands of NAND gates!