You magically distract the triggering creature and turn its momentary uncertainty into encouragement for another creature. The triggering creature must reroll the d20 and use the lower roll.
You can then choose a different creature you can see within range (you can choose yourself). The chosen creature has advantage on the next attack roll, ability check, or saving throw it makes within 1 minute. A creature can be empowered by only one use of this spell at a time.
Spell Breakdown
Any time you see a creature within 60 feet succeed on an attack roll, ability check, or saving throw, Silvery Barbs requires them to roll another d20 and take the lower of the two numbers. In other words, you can give disadvantage after the target has already succeeded. This is a powerful reaction spell that could upend everything from combat to social interactions.
The wording here is important, because it is not technically disadvantage itself. That means you can force another d20 roll for creatures that have already rolled with disadvantage.
This use of the spell alone is excellent, but there’s more to it. When casting this spell, you also give an additional creature (including yourself) advantage on the next attack roll, saving throw, or ability check.
* - Any time you see a creature within 60 feet succeed on an attack roll, ability check, or saving throw, ** - (powdered rhubarb leaf and an adder’s stomach)