A jackpot is the largest prize available in a game. In a fixed jackpot the amount stays the same, and in a progressive one a small part of each bet is added to the prize, so it keeps growing until it is won. Large jackpots are very rare. They are paid for by the many smaller losses of everyone who plays, and a bigger jackpot does not improve the odds of winning it. In slots, a share of the return to player is often spent on the jackpot, which can make ordinary wins smaller.
Example
A progressive jackpot starts at 10,000 and gains 1% of each bet. After 2 million units are bet in total, it has grown by 20,000 to 30,000, if no one has won it.