Net winnings are what you actually gain from a bet: the total amount paid out minus the stake you put in. Stake 20 on a bet that returns 150, and the total payout is 150 but your net winnings are 130. Put simply, net winnings are the extra money a bet earns beyond the stake you risked.
The difference matters because many figures in gambling are gross. A payoff is the full amount returned, stake included, and decimal odds also show total return per unit staked. Net winnings take the stake back out. The term means the same thing in a casino, at a sportsbook or anywhere else. Operators work with the same logic at scale: gross gaming revenue is total stakes minus total winnings paid out.