You technically only mark to mark futures contracts, not forwards.
When you mark to market you actually settle the gain or loss of the contract throughout the period. So if on Monday you had a $10 gain, that night the gain is settled and there is technically an additional $10 in your account. If on Tuesday it loses $20 then it is taken out of your account. This is why futures require margin accounts so they can make sure you will meet mark to market requirements.
Now if this were a forward contract, you wouldn’t mark to market. Instead you would calculate the total value up until this point. So if we use the same example as above…. On tuesday the value of the forward contract would be the combined gain of 10 and loss of 20, equating to $-10 (I didn’t include discounting or anythign like that in order to simplify the example). Even though you have a -$10 you don’t actually have to pay out the $10 like you do on a mark to market futures contract.
This is why American options are more valued than European options for futures contracts when compared to forward contracts.
If the futures contract has a gain of 1,000 and you execute the american option, you immediately get the contract, mark to market the $1,000 gain and can earn interest on your $1,000. While with a European option you cannot exercise early. As with forwards the American option is considered to be equally valued because you cannot mark to market a forward and thus if you had a $1000 gain on a forward it wouldn’t matter as you would still have to wait until the VERY END of the forward contract to realize the gain and start earning interest on it.
Hope that helps and is clear enough