Okay kiddo, the Congressional Budget Office is a group of very smart people that help the United States government figure out how to spend money. They use special math skills to figure out how much things cost and how much money the government has.
They try to make sure that the government doesn't spend more money than they have, because that could be bad for the country. They also try to make sure that the government spends money on things that are really important, like helping people who need it or improving roads and bridges.
When important people in the government want to make changes to how money is spent, they go to the Congressional Budget Office to get advice. The people at the Budget Office tell them if their ideas will work and if they will cost too much money.
So, the Congressional Budget Office is kind of like a group of grown-up nerds who help our government use money wisely and make good decisions that will help our country.