Mark price is the value the exchange assigns to your position for margin accounting and liquidation triggers. It is computed from an index of prices across major spot markets, usually with a funding component and a short moving average layered on, and it updates continuously.
01Why not the last trade
Last traded price lives on one order book. A single large market order can print a wick that has nothing to do with where the asset trades anywhere else. If liquidations triggered on last price, any participant with size could harvest stop clusters by pushing one book. Mark price, built from external spot indexes, does not move on one exchange's tape, so a wick on the derivatives venue itself does not liquidate anyone.
02Reading it in practice
Your unrealized P&L is usually shown in both mark and last terms. Margin ratio and liquidation distance follow mark. During fast moves mark lags last, which is why a position can look liquidated on the chart's wick yet survive, or survive the wick and die a minute later as the index catches up. In our liquidation feed, event timestamps are the exchange's engine time, and the same lag explains small gaps between chart wicks and the liquidation burst that follows them.