This would be analogous to your spending more money than you have in your checking account. The national debt is the money owed by the government to people who loan it money to operate. To pay for things it needs or wants, but they don't have the money. But, there's another side to this, and it's not talked about very much because the issue gets complicated. First, let's look at the debt itself.
Most people carry some kind of debt.
Consider buying a home. Ultimately, it comes down to how much income you have and the amount of the monthly payments for a mortgage. While we want people to be able to buy homes, we also want to make sure the homes are worth it. Here, they own assets they would not otherwise be able to have. This not only makes the quality of living better for most Americans, but also is a great part of our economy.