Falloff describes a decrease in light level, commonly as distance from a source increases or as a subject moves away from the brighter part of its beam. Specify which kind of change you mean when planning or comparing setups.
Distance and beam shape
The inverse-square law is a useful distance model when the source behaves approximately like a point source. It does not describe every close-range softbox, panel or focused optical system accurately. A separate change across the beam can come from its angular distribution, even where the distance remains similar.
Practical example
Compare two illustrative point-source arrangements. A person moving from 1 m to 2 m receives one-quarter of the original illuminance, a two-stop loss. Moving from 4 m to 5 m gives about 64% of the original illuminance, roughly two-thirds of a stop less. The same one-metre movement has a different effect because the relative distance change differs.
Using falloff in a scene
A close source can create a strong exposure difference between a foreground subject and a more distant background. Moving the source farther away can reduce that relative difference, but may require more output and change its apparent size. Meter the actual marks: walls, ceilings, modifiers and overlapping lights can flatten or complicate the expected falloff.
Related terms
Inverse-Square Law · Photometrics · Stop · Feathering · Soft Light
References
IES: inverse-square law and its limits · ARRI: fixture photometric data