The full market value of what a position controls, regardless of how much cash was put up to hold it.
Margin and leverage. A $5,000 deposit can control a $100,000 position, which is 20:1 leverage. Because the loss is measured on the full $100,000, a 2.5% move against you halves the deposit and brings a margin call, and a 5% move uses all of it.
Notional value is contract size times price. It is the number that tells you your true exposure. A leveraged position can have a small margin requirement but a very large notional value.
Traders who size by margin instead of notional are the ones who get surprised. A 1% move happens to the notional, not to the margin.
Example: one es contract at 5,000 has a notional of $50 x 5,000 = $250,000. If the day-trading-margin is $500, a 1% adverse move (50 points, $2,500) is five times the margin you posted.