Okay, imagine you and your friend are playing a game and you both made up a rule so you know what to do in the game. But, if there was a problem and you didn't agree on what the rule meant, you might ask an adult to help figure out what to do. This is kind of like what happened with Swift v. Tyson.
Swift v. Tyson was a court case a long time ago in the United States where two people had a disagreement about what a law meant. One person thought the law meant one thing, and the other person thought it meant something else. They went to the court to ask for help to settle the disagreement.
Now, in the United States, there are different court systems. One system is the federal court system where they follow laws made by the whole United States government. Another system is state court systems which follow laws made by the government of the state where the court is located.
In Swift v. Tyson, the disagreement was about a law made by the state of New York. But, the court decided not to follow New York state law and instead made a decision based on what they thought should be the right answer. This might seem strange, like if your friend made a different rule during the game, and an adult decided to make up a new rule instead of following the one you both made up.
Some people think this wasn't fair because it caused confusion about what laws should be followed in different places in the United States. So, after a while, the court system decided to stop using this decision as an example for future cases, and instead said they would follow state laws when making decisions in state courts.