Hey kiddo, do you know what a Rotation Period is?
Well, when you spin around and around, the time it takes you to complete one full turn is called a Rotation Period.
Now, imagine the Earth spinning around just like you do. It also has a Rotation Period, but it takes much longer because it's much bigger than you. It takes 24 hours for the Earth to completely rotate once, and that's why we have day and night.
Other planets, like Mars or Jupiter, also have their own Rotation Period, but they might be longer or shorter than Earth's. Some planets even rotate so slowly that it takes them months, or even years, to complete just one rotation!
That's basically what Rotation Period means - how long it takes an object, like a planet or even you spinning around, to complete one full rotation. Cool, right?