Last price
The last price is the price of the most recent trade on an exchange. It is what the chart shows and what most stop orders follow by default, but liquidations follow the mark price.
Every trade sets a new last price, so it can jump in a sudden wick even when one large order hits a thin book. The mark price smooths those jumps with prices from other venues. Your realised profit and loss is counted at the last price you traded; your unrealised profit and loss and your liquidation use the mark price.
Learn more: how to avoid liquidation.