Nonlinear gameplay means that you can play a game in many different ways, instead of just following one path. Imagine you have a toy car, and you can drive it anywhere you want in your playroom. You can make it go left, right, or straight, and you can also stop the car or turn it around. That is kind of like nonlinear gameplay!
In a video game, nonlinear gameplay works the same way. Instead of being told what to do next, you get to make choices. You might have a bunch of different places to explore, and you can decide which one to go to first. You might have different objectives to complete, and you can choose which order to do them in.
Nonlinear gameplay gives you more freedom and control over your experience. It also makes the game more replayable, because you can try different choices and paths each time you play. Overall, it's a fun way to play a game!