Players buy Robux with real money or a subscription and spend it on items, passes and in-experience purchases. When a creator sells something, the earned Robux lands in their account, after Roblox's platform fees are applied.
Eligible creators can then use the Developer Exchange, or DevEx, to convert earned Robux back into real currency, subject to age, account, balance and verification requirements. This loop, purchase to spend to earn to cash out, is what turns games into a real business on the platform.