Every transaction has two sides, so the accounts sum to zero apart from statistical error. A country running a current-account deficit is necessarily importing capital to fund it, whether through foreign purchases of its bonds, direct investment, or a drawdown of fx-reserves.
That identity is the useful part for a currency trader. A deficit is not in itself bearish; what matters is the quality of the financing. Long-term direct investment is stable money. Short-term portfolio inflows chasing a yield gap can leave in a week, and a currency funded that way is fragile to a change in the interest-rate-differential.
Crises typically appear as a sudden stop, where the inflow that was funding the deficit simply ceases and the exchange rate has to do the adjusting.
Example: a country runs a current account deficit of 5% of GDP funded by portfolio inflows. If those inflows halve, the currency has to fall far enough to compress imports and attract the rest, which is rarely an orderly process.
Related: current-account, capital-account, fx-reserves, emerging-market-currency