The gamble feature on slot machines is a feature created to give you a chance to build on your prize after a winning spin. Basically what the feature boils down to is a coin flip, a 50/50 chance at doubling up your winnings. Guessing correctly will your double your winnings. Additionally, many gamble features give you the opportunity to keep gambling your winnings, moving from 2x, to 4x, to 8x your winnings, and so on.
Slots Gamble Feature - Microgaming
There are many online slot developers with this feature enabled as it has become quite a hit with players because of the potential for easy double up winnings. Some of the popular software developers with this feature are Microgaming, Cryptologic, and NetEnt.