An expanding wild is a wild that stretches to fill every position on its reel. A single wild in the middle row becomes three stacked wilds on a 5x3 game, covering more paylines or ways at once.
Some expand whenever they land; others expand only when doing so creates a win. Many games pair the expansion with a free respin, during which the expanded reel stays wild while the others spin again. Some also carry a multiplier.
A full reel of wilds is a strong position because every line through that reel benefits. Expanding wilds are built into the game's maths, so their value is already reflected in the published RTP. See the expanding wilds feature page.