Fall-off depends on the distance from the source of light.

For relatively small light sources, the amount of illumination an object receives follows the inverse-square law. That means that an object that is twice as far away from the light source as another object receives only one-quarter of the amount of light.

Outdoors, in bright sunlight, the difference in distance between any two things on Earth and the sun is negligible. The farthest two things on Earth can be from one another is only about 13 thousand kilometers (and no two things in a picture taken on the Earth can be that far apart), and the distance to the sun is about 150 million kilometers. You would hardly be able to measure, let alone see, the difference in the brightness of the light falling on objects whose difference in distance from the light source is less than one tenth of one percent. On overcast days the light source is the clouds (the sun still provides the light, of course), and they're still too far away from your subjects for fall-off to be a concern.†

Indoors, with the source of light much closer to one subject than to another, you will be able to see (and photograph) a difference. That makes lighting indoor group portraits tricky -- you either have to use very powerful lights from far away (if you have the space) or find a way to make the distances between the lights and each of the subjects more-or-less the same. (Or, with a bit of artistic storytelling, you can "feature" some subjects at the expense of others.)

Using very large light sources can help; within a certain distance of a large source, light will fall off in a linear fashion (things twice as far away get half as much light) or even more gradually.

On the other hand, sometimes fall-off is exactly what you want, and taking advantage of it by putting the light source very close to your subject will enhance the effect.

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† For landscapes at dawn or dusk, you may find that you can see fall-off if clouds are the main source of illumination, but that's likely to be the reason you're taking the picture and not a technical problem to deal with.