A margin call is the warning stage before a stop-out. Your broker sets a margin-level threshold — often 100% — and when your equity-to-used-margin ratio drops to that level, the platform flags the account and typically sends you an email. You can respond by depositing more funds or closing losing positions.
If you ignore it and the market keeps moving against you, the broker hits the stop-out level (often 50% or 20%) and starts closing your largest losing positions automatically. Modern forex brokers enforce negative balance protection for retail clients in regulated jurisdictions, so your loss is capped at your deposit — but margin calls still cause avoidable, sudden position closures.