The government spends more than it receives (from taxes and fees). To get the extra money, they sell “bonds”, which is a certificate that says you gave the government money, and they promise to pay it back in a certain amount of time, with interest.
The money the government owes people / companies / governments who bought bonds is national debt. When the government gets tax money, a portion of that money goes to whoever bought bonds.
People buy government bonds because the government is good about paying the money back, with interest. The government doesn’t pay a lot of interest, but they’re reliable so it’s a very safe investment.
Americans buy a lot of US bonds.
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