Colossal symbols are single large symbols that occupy a block of positions across more than one reel and row, for example 2x2 or 3x3. Each position the symbol covers counts as that symbol for working out wins.
A 3x3 colossal symbol on reels two to four of a 5x3 game fills the middle of the grid, so a single matching symbol on reel one can form many winning ways or lines. That makes colossal symbols a source of occasional large wins.
They differ from stacked symbols, which are separate symbols that happen to line up vertically on one reel. Some games use colossal wilds, and some combine them with a "giant reel" layout in features. As with any mechanic, the value is part of the published RTP.