The federal government spends more than it collects most years. The gap is a deficit; the accumulated total is the national debt. Nearly everyone agrees the trajectory matters, and almost no one agrees on which combination of spending cuts, tax increases, or growth should close it.
A deficit is a single year's shortfall. The debt is the sum of past deficits, held by domestic investors, foreign governments, the Federal Reserve, and federal trust funds. Interest on that debt is itself a growing line in the budget, and it competes with every other priority.
Most federal spending is not set annually. Social Security, Medicare, Medicaid, and interest run on autopilot under permanent law and consume the majority of the budget. Annual appropriations, which include defense and most domestic agencies, make up a smaller share than most people assume.
Economists distinguish between the raw dollar figure and debt as a share of the economy. A debt that grows more slowly than GDP is generally considered sustainable even if the dollar total rises. Disagreement centers on where the danger threshold sits and whether one exists at all.