Hold and win — also branded as respin, link-and-win, or lightning-style features — triggers when enough special coin or money symbols land at once. Those symbols lock in place, and you are given a small number of respins (often three) that reset each time a new special symbol lands. When respins run out, the collected values, and sometimes a jackpot, are paid.
What actually decides the win
The size of the result depends on how many money symbols you collect and the values printed on them. The respin counter and the "so close" near-misses are presentation; the outcome is drawn by the RNG exactly as any other feature. See hit frequency for why the near-miss feeling is engineered.
Where it comes from, and the four jackpots
The mechanic is a land-based import: Aristocrat's Lightning Link cabinets (2015) made "hold and spin" the most copied feature on a casino floor, and the online versions from Playson, Booongo, Pragmatic Play and others keep its furniture — a trigger of six coin symbols on a 5×3 grid, three respins that reset on each new coin, and a ladder of four fixed jackpots usually labelled Mini, Minor, Major and Grand.
Those four are worth reading as multiples of the stake rather than as the cash figures on the meter. Mini and Minor are commonly a few tens of stakes and land often enough to feel routine; Major sits in the low hundreds; the Grand — often a thousand stakes or more — is normally paid only for filling every cell on the grid, which means collecting fifteen coins with only three respins in hand at any time. That is a very small probability, and it is the reason the Grand can be quoted so large while the average feature pays a handful of stakes.
The coin values themselves are weighted the same way: the smallest denominations are by far the most common draws, and the coins that carry a jackpot symbol are the rarest. A feature that "nearly" filled the grid did not nearly pay the Grand; the last cells are exactly where the draw is thinnest.
Reading it
These features usually sit at medium-to-high volatility. The jackpots quoted are ceilings reached rarely — read the paytable for how the fixed jackpots relate to your stake before you assume the top prize is in reach.