Okay kiddo, imagine you're playing a game with your friends. Let's say you're all taking turns hitting a ball with a stick, trying to see who can hit it the farthest.
Now, imagine that before the game starts, you all agree on a rule called "one strike, you're out." This means that if you swing at the ball and miss it, or you hit it but it doesn't go very far, you're done playing for that turn. You have to sit out and watch the rest of your friends take their turns until it's your turn again.
This might not seem fair at first, but it actually makes the game more exciting and challenging. It means that you have to really focus and try your best every time you swing at the ball, because you only get one chance to make it count. It also means that the game moves faster, because there aren't any players who just keep swinging and missing over and over again.
So, "one strike, you're out" is just a rule that makes games like baseball or softball more interesting and competitive. It means that you have to be careful and make every swing count, and it keeps the game from dragging on forever.