How does the US keep buying more from other countries than it sells to them?
Background
The balance of payments is an accounting framework before it is anything else. Every transaction has two sides, and the full record closes only after the financial account, reserve assets, and errors and omissions are included. That makes it easy to misread: BOP crises show up in the composition of financing and in stocks of external debt, not in a headline accounting total.
The economic content comes from asking what kind of capital is financing the current account and whether that financing is sustainable. Short-term debt financing a consumption boom is a different problem from long-term FDI financing productive investment, even if both produce the same headline deficit.
What it covers
The balance of payments is a systematic record of all economic transactions between a country's residents and the rest of the world over a given period. It separates the current account, capital account, financial account, reserve assets, and errors and omissions.
A current account deficit is financed through net borrowing from abroad, asset sales, reserve movements, or measurement discrepancies. The accounts reconcile as a full system; the policy read depends on the reason for the deficit and the way it is financed.
Open question
What does a current account deficit actually mean, and when should it concern policymakers?