Updated: Jul 15, 2020
"Last Price" is the last price of any trade that takes place at BitMEX platform. We count even 1 USD value trade. The important guide is that "Mark Price" (fair price) used on BitMEX platform is a price computed as average price from tree other cryptocurrency exchanges: Kraken, GDAX (Coinbase), BitStamp + order books. Why do we need to use two different prices?
The reason why we have Last Price and Mark Price at Bitmex? To Avoid price manipulations. Imagine the situation when you are shorting and somebody else clicked with a fat finger to buy 10,000,000 contracts instead of 100,000 Let say just by mistake.
The average Bitcoin price on the main exchanges is in 8000 - 8010$ range.
Bitmex Mark Price = 8005,5$
Bitmex Last Price = 8007,5$
You are short and your liquidation price is 8030$. You are about to close it by clicking Buy Market but some fat finger at Bitmex has just brought 1.000.000 and it dramatically increased Bitmex last price up to 8335$ (for one second) and after while the last price is 8008$. Notice that 8335$ is higher than your liquidation price and it means your position would get liquidated. It would but not will only because Mark Price protected your trade. Generally, Mark Price is in trader's favor. The point is that Mark Price is more independent than the Last Price.
All the above is about Bitcoin and Ethereum Perpetual contracts. Computing Mark Price for a futures contract is a little more complicated as there is value in time added as well - look more guides and tutorials on MinistryOfMarginTrading.