The jackpot is triggered by a specific combination of symbols on the reels. When you pull the lever or push the button, the random number generator generates a mixture of symbols. If the given combination matches the jackpot combination, you win big time.
Good bankroll management is one of the most important tips for winning on slots. Bet too much and you risk going broke before you have a chance for the luck to even out. Bet too little, however, and you risk not maximizing your profits. It's important to plan your stakes correctly.