Okay kiddo, so defamation is when someone tells lies about another person that harms their reputation. Like if someone spreads a rumor that you stole from the store when you didn't, that would be defamation.
But in the United States, we have laws that protect people's reputations from being damaged by false statements. So if someone does say something untrue about another person that hurts their reputation, the person who was harmed can take legal action against the person who made the false statement.
It's important to remember though, that not everything negative that is said about someone is considered defamation. If it's true, or if it's an opinion (like if someone says they don't like your new haircut), it's not defamation. Only false statements that harm someone's reputation can be considered defamation.
So in summary, defamation is when someone tells lies about another person that hurts their reputation, but in the United States there are laws that protect people from being defamed and they can take legal action against the person who made the false statement.