It's where the roll indexes a results table, which can be fixed, sliding, dynamic, uniform, bespoke etc.
Example of a 'fixed' roll index (simplified FitD)
Highest die is 6, it’s a full success—things go well.
Highest die is a 4 or 5, that’s a partial success—you do what you were trying to do, but there are consequences: trouble, harm, reduced effect.
Highest die is 1-3, it’s a bad outcome. Things go poorly. You probably don’t achieve your goal and you suffer complications.
That's a fixed index. One virtue is that it's both nuanced, but also easy to remember in play. A roll that isn't an index is dealing damage in 5e. Roll d6, get 4, deal 4+STR damage e.g. 7.
Example of a 'sliding' roll index (simplified 5e)
DC 17
Modified roll is 17+, it's a full success
Modified roll is <17, it's a failure
With the latter, the number associated with success always changes. There's a lot more to both these example systems. But anyway, what I've done in my homebrew is translated the sliding index into a fixed index, so that we can focus better on nuanced outcomes. Then, because we're focusing more on the outcomes, possibly you see how that can work well for "consequences-resolution."