Okay kiddo, have you ever heard of a country called the United States of America? People also call it the USA. Uncle Sam is like the symbol or mascot of the USA. He's a man with a white beard, a top hat, and wears red, white, and blue clothes.
Uncle Sam is named after a man named Samuel Wilson who used to work for the government in the early 1800s. He worked at a place that made food for the soldiers. They put his initials, "U.S.", on the boxes to show that the food was from the United States. People soon started to call the food "Uncle Sam's" because of the initials, and then they started to call the government "Uncle Sam" too.
Now, when we think of Uncle Sam, we think of the government and the country he represents. He reminds us to be proud of where we come from and to do our best to be good citizens.