Think of them like store vouchers, only they're used within Roblox itself. Each gift card provides a certain number of Robux (the game's currency), and these are spent on additional in-game ...
Players can buy Robux from the Roblox website or through a gift card. $0.99 equals 80 Robux. Roblox was first released in September 2006, making the game older than most of its user base.