An alarm input is a terminal on a panel or controller that watches a sensor circuit and reports its state: normal, open, shorted or in alarm. Door contacts, motion detectors and glass-break sensors all land on alarm inputs, so the number available shapes what a site can monitor.
An alarm input is the smallest unit of detection in a panel. Whether it is wired well and supervised properly decides whether the system notices a fault or simply misses a sensor that was cut.
Zones, circuits and supervision
A sensor is wired normally closed or normally open, and an end-of-line resistor lets the panel tell a healthy circuit from one that has been opened or shorted. That supervision is what separates a cut wire from a quiet room, and it is why the resistor belongs at the far end of the run.
Inputs, outputs and purpose
An input reports; an output does something, such as sounding a device, releasing a lock or calling a camera preset. Wiring a sensor to the wrong terminal produces a fault that looks like a programming problem, so a point list naming every input and its purpose is worth having.