Cash on reels describes symbols that show a prize value on their face, usually as a multiple of the stake or a cash amount scaled to it. Instead of paying through lines, they pay when a feature collects them.
They are the building block of hold and win rounds, where they lock in place, and money collect mechanics, where a collector gathers them. Some include jackpot labels instead of a value, linking them to fixed jackpots.
Seeing large values on the reels without a collector can feel like a missed win, but in maths terms most cash symbols on most spins are worth nothing until a feature uses them. Their values and how they pay are in the game's paytable.