What does Marked to Market mean?

Marked to Market is an industry standard accounting method of keeping track of the Profit and Loss (P&L) on your overnight or long term position(s).

The closing price at 4 PM ET is used to determine the unrealized profit or loss on your position. This amount is added or subtracted from your equity daily.

This method is also used if you enter a position after 4 PM ET.

You can read about this method further using the following link: https://www.investopedia.com/terms/m/marktomarket.asp