Feathering means aiming the light so your subject is lit more by the edge of the beam or modifier than by its center. The effect is usually not that the source becomes physically larger, but that you use a gentler part of its light pattern.

With many modifiers, the center is brighter and more direct, while the edge has a smoother transition from highlight to shadow. Lighting with that edge can make shadows appear less abrupt and can create a more gradual tonal falloff on the subject.

It also improves light placement: if a softbox is pointed straight at the subject, a lot of its output may spill past or behind them. Turning it so the subject sits in the edge of the beam can use the modifier more efficiently and control where the brightest light lands.

This works best with directional modifiers that have a defined beam edge, like softboxes and beauty dishes. It is usually less obvious with umbrellas, which spread light more broadly and have a less distinct edge. Small bare flashes are also less effective unless used with modifiers.