Balance of Payments

“Thinking in this way. any deficit in the current account must be made up by a surplus in the combined capital account. That is. the excess of imports over exporlS must be offset by sales of assets and debt incurred to foreign entities, A current account surplus is similarly offset by purchases of foreign physical or financial assets.” I do not get why the current and capital accounts have to cross eachother out. How does this relate to the definition of the balance of payments?