A Bonus Game is a special reward stage that activates when a specific condition is met (usually the appearance of 3 or more 'Bonus' symbols), transitioning from the spinning reels to a completely new and separate game screen.
This is different from features like free spins, where new rules are added to the existing reels.
The core of a bonus game is to provide a new experience, as if you are enjoying a 'mini-game within the game', completely breaking away from the basic slot gameplay.