A Contract takes 3 to 10 skins from your inventory and returns a single one worth between 75% and 125% of their combined value. The page shows you that minimum and maximum before you confirm, and the skins you put in are consumed either way.
It is a reroll around what you staked, not a guaranteed step up: the result can come back worth less than the sum of its inputs. That is the honest way to describe it, and it is why the range is printed rather than buried. The roll is provably fair like every other roll on the site, so you can recompute it afterwards from your own client seed.
Valve's in-game Trade Up Contract is a different mechanic that happens to share the name — it takes exactly ten skins of one rarity and returns one from the tier above. If you are looking for that, the CS2 trade up contracts guide covers the rules, the collection maths and how the output float is calculated.
Every roll on DuckSkins is provably fair and independently verifiable, items can be withdrawn as real CS2 skins to Steam, and support is on hand 24/7. It's built for players who love the thrill of opening a case and watching their collection grow.