One kind of interrupt is a hardware interrupt. A phyical (hardware) stimulus is imposed on the CPU. It takes the form of a voltage on a wire, same kind of signaling as flipping a switch to turn something on. (Since that too is a voltage on a wire to trigger its effect.) The wire (called a pin) in this case is a particular wire emerging from the CPU. It is one of various wires, each with its own purpose. This one is provided specifically to offer a way to “interrupt” the CPU, or “yank its chain” from the outside.