Watta satta is when two families agree to marry their children to each other. It's like a promise that the boy and girl will get married when they grow up. This happens in some cultures where families have a lot of importance in getting married.
It's like trading two things. The boy's family agrees to give their son to the girl's family to marry, and the girl's family agrees to give their daughter to the boy's family to marry. This is called "exchange marriage," which means giving something in return for something else.
But, it's not a good thing because it means that the boy and girl don't really have a choice in who they get to marry. They have to marry the person their family picks for them. It's important to remember that everyone should have the choice to marry who they want to, and not just who their family tells them to.