Stacked symbols are groups of the same symbol placed one after another on a reel strip, so when they land they occupy several positions on the reel, sometimes the whole reel.
Stacks are a volatility tool. Most spins see little of them, but when stacks of the same symbol line up across several reels, many paylines or ways pay at once. Stacked wilds work the same way, substituting across a full reel.
Stacks are different from colossal symbols, which are single oversized symbols spanning several reels and rows. On a stacked reel each position is still a separate symbol.
Many games make stacks more frequent during free spins, which is part of why features pay more. The effect is built into the game's published RTP.