Vaal Orbs are a type of currency you find from Act 3 onwards. If you need a briefing of how currency works, check out our PoE 2 Currency Guide.
The Vaal Orb is an extremely high risk crafting orb, with RNG outcomes that can be good... but that can also be awful.
Using a Vaal Orb on an item permanently prevents that item from having other currencies used upon it. If you haven't applied quality, filled all modifier slots with Exalted Orbs and applied all possible rune sockets before Vaaling the item, you never can.
A Vaal Orb can boost an item beyond normal limits, but it can also ruin the item, rerolling high rolls into low ones, replacing valued modifiers with useless ones. Or it can achieve nothing besides adding the 'Corrupted' tag.
It is at its best when you have two very similar items, such as two weapons that are within 1-2% damage from each other. If you Vaal one and the Vaal Orb rolls well, it may pull 5% ahead.
And if you roll poorly... well, you had a backup for a reason.