Mark to market in finance means that the value of a position is estimated by using the market price of an asset. This practically means that a futures position is being valued every day based on the clearing house assessment of the price and the result is either loss or profit for the trader, which is taken or deposited to his/her account accordingly. So, every day we have real transferring of money among the accounts of market participants, something that doesn’t happen in options, stock markets or forex.

Suppose that we are long 1 future contract of WTI crude oil at $100/barrel. The value of the position the moment of the opening is $100,000 ($100/barrel x 1000 barrels/contract). If the future price at the end of the day is $98/barrel, then we have a loss of $2/barrel which totals to $2,000. This money is being transferred from our margin account to the margin accounts of those who have short positions in the future.