You use this soft currency to purchase various upgrades that increase the rate of that soft currency increasing, often exponentially. Each game has a different currency: Coffee Corp has straight cash, Egg Inc.
If you’re uninitiated in this world, an idle game is one in which an in-game soft currency constantly increases, even when you’re not playing.
Idle games (sometimes called incremental games) are a fascinating genre all of their own, from a psychological perspective, design perspective, and – naturally – a business perspective.