Copyright is a way of making sure that people are legally allowed to use a creative work like a book or a song. When a work is copyrighted, its creator owns the rights to that work and can choose who can use it and how they can use it.
The federal government of the United States creates lots of creative works like documents, websites, art, and music. Most of these works are protected by copyright. That means the government owns the rights to them and gets to decide who can use them. Usually, the government allows people to use its works free of charge, but they must follow certain rules. For example, they might only be allowed to use the work for non-commercial purposes and give the government credit for creating it.