What It Means
MTM (Mark to Market) is revaluing your open positions to their current market price — rather than waiting until you actually close them — to calculate real-time unrealized profit or loss.
How It Works
For F&O positions specifically, MTM losses and gains are settled in cash every day: you have to fund a loss on a position you're still holding, even though you haven't closed it. For intraday equity, MTM shows up on your dashboard as unrealized P&L that only becomes real the moment you square off.
Warning
An MTM loss on a still-open F&O position is real money leaving your account daily, not just a number on a screen — don't confuse it with an equity holding's unrealized loss, which isn't locked in until you actually sell.