Mark To The Market is the monitoring of a the current value of a position relative to the price at which the trade was executed for securities purchased on margin or on a when issued basis.
Applying "Mark To The Market" to Securities Exams:
When a customer purchases a security on margin the customer is borrowing money from the broker dealer to purchase the securities. When a brokerage firm lends a customer money to purchase securities on margin the broker dealer’s capital is at risk. As such the broker dealer will continuously monitor the position to ensure that its interest in the postilion (its loan) is protected.