Specifically, the national debt is a term referring to the level of federal debt held by the public, as opposed to the debt held by the government itself. Because the U.S. government almost always spends more than it takes in, the national debt continues to rise.

Though the national debt can be measured in trillions of dollars, it is usually measured as a percentage of the gross domestic product (GDP), the debt-to-GDP ratio. That's because as a country's economy grows, the amount of revenue a government can spend to pay its debts grows as well.

In addition, a larger economy generally means the country's capital markets will grow and the government can tap them to issue more debt. This means that a country's ability to pay off debt — and the effect that debt might have on the country's economy — is dependent on how large the debt is as a proportion of the overall economy, not the dollar amount.

For an expanded discussion of the national debt, you will want to read this article.