Premium money is the money put down to attain or secure insurance. Within the gaming world, players are enabled in certain games to reserve some coveted features through payment, either through virtual money or real life transactions.
To entice players to spend money as well as motivate them to continue gameplay, certain games often put up deals and reduced prices on features that are coveted by players. Be it extra lives or extension packages of gameplay; the players will then use their money to put down payments to access the deals.
These deals can be either obtained as in-game purchases using the currency system of the game that has been collected by the player or via real life money that the player will deposit to the game developers' accounts before gaining access to the features. Often these features are available in a smaller quantity as in-app purchases but provided to the player in a larger bulk deal online via real currency.
The money put down by the player as payment for the coveted feature is the premium money technique used by the game developers.