You can see the word "part" inside the word "participate." To participate in something is to take part in it. In other words, participation is doing something with other people. It's getting involved, and playing or working or somehow taking part in what the other people are doing.
When you participate in a game with your friends, you're playing the game with them. When you participate in a project with your family, you're helping them with the project. And when you participate in a conversation, you're one of the people who's listening and talking.
You may not always want to participate. This kid has no interest in participating in this activity. The other kids are participants: that means, they're the people who are doing the activity. But this kid does not want to be a participant.
You can participate in class by raising your hand a lot. You can participate in a contest or a debate by joining in and doing your best. But if your siblings are fighting with each other, or if your friends are starting a food fight, you might choose not to participate. You could say, "Nope. Leave me out of this!"