Progressive Jackpots are Jackpots that increase in value the more the game is played. The more players that play the same slots game, the bigger the Progressive Jackpot will be, until of course - a lucky player wins it!
When playing a slots game that has Progressive Jackpot there is a required bet amount needed before you are eligible to win the Progressive Jackpot. Some slots games have multiple Progressive Jackpots (mini, minor, major, mega and grand) and to eligible for all the player needs to bet high enough, which also increases their chances of a win!
When opening a slots game that has a Progressive Jackpot, the player will see the "select your bet" screen, which offers two choices; the minimum eligible bet and the higher bet for better chances.
However, you don't have to play the minimum amount to enjoy the slots machine, but it will mean you will not be able to win any of the Progressive Jackpots (which can be seen darkened in the image above).
If you accidentally lowered the bet below the minimum amount, you can increase it again to become eligible once again for the Progressive Jackpots.