The US Government spends more money than it takes in taxes. In order to finance the difference it takes out loans (in many ways just like you might take out a loan to buy something). These loans take the form of government bonds.

Various people/organizations buy these bonds and give the US government money. Later on these bonds "mature" and those people get their money back plus some interest.

This post is saying that about 9 trillion dollars worth of these bonds will mature in the next 4 years so the US government will need to come up with that much money to pay the bondholders. The way the government does this (for the most part) is by "rolling over" the debt. This means the government will issue new bonds that pay out at some point in the future.

In some ways this is actually great for the government because interest rates are at very low levels right now, which means the new bonds will (in most cases) have a lower interest rate than the old ones.