Sticky wilds are wild symbols that stay locked in place on the reels for one or more further spins, instead of disappearing when the reels spin again. While they stay put they keep substituting for other symbols, which makes the following spins more likely to land a winning combination.
Wilds themselves are almost universal: apart from games built to look like vintage machines, nearly every modern video slot has them. Sticky wilds have exactly the same purpose; the only difference is how long they stay. That depends on the game. Many are tied to another feature, remaining for the whole of a free spins round in one title, or only until a sequence of cascading reels ends in another. The paytable explains exactly how they behave.
Sticky wilds in your game mix
Sticky-wild features usually raise the value of a bonus round, so they are popular anchors for free-spin promotions. Before building a campaign around a title, check whether its sticky wilds appear in the base game or only inside features, since that changes how quickly players see them.