Day's P&L shows how much your holding's value has moved today — it's calculated as your current price minus the previous day's closing price, multiplied by your quantity. This is different from your overall P&L, which is your total gain or loss since you bought the holding.
How is Day's P&L calculated?
Day's P&L = (Current price − Previous day's closing price) × Quantity held
For what counts as the "previous day's closing price," see Why does my P&L change after 3:30 PM? — it depends on whether the stock has F&O contracts.
How is this different from my overall P&L?
Overall P&L = (Current price − Your average buy price) × Quantity held
Overall P&L is your total unrealized gain or loss since you first bought the holding, however long ago that was. Day's P&L only reflects today's price move, regardless of when you bought it or at what price.