Okay kiddo, have you ever been to the beach and seen the big blue ocean? Freedom of the seas is like making sure everyone can use the ocean and sail on it without anyone telling them they can't.
Imagine if someone said you couldn't swim in the ocean or you couldn't play in the sand. That would be pretty sad, wouldn't it? The same thing can happen with big boats and ships that go around the world carrying things like food, toys, and clothes. They need to be able to move freely on the ocean, without any country saying they can't.
That's why a long time ago, countries got together and agreed that all the oceans in the world should be free for everyone to use like a big playground. But just like we have rules at the playground to make sure everyone plays nicely, there are rules for using the ocean too. These rules are called international laws that every country needs to follow.
So freedom of the seas means that everyone can sail around the big ocean and nobody can say they can't. It's like a big playdate for big boats from all over the world to come together and trade toys, food, and clothes.