Insurance fund
An insurance fund is money a futures exchange keeps to cover losses that go past a liquidated position's margin, so they do not fall on the trader's other funds or on other traders.
In a fast move or a gap, the exchange may close a liquidated position at a worse price than its liquidation price. On isolated margin your loss still stops at the position’s margin: the difference is covered by the exchange’s insurance fund. Binance and Bybit both run one. If the fund runs short, some exchanges reduce winning positions on the other side (auto-deleveraging).
Learn more: isolated vs cross margin.