Creator Credits, or simply credits as often referred to in your monetization dashboard, is the currency earned by creators from the content they’ve sold in the marketplace. It can be either used to purchase content on the marketplace (for games using the mod.io marketplace with virtual currency), or paid out in USD.
When paid out, Creator Credits have a fixed value of $0.004 per credit, meaning that 250 Creator Credits are worth $1. See here for details on payouts.
When used to buy content on a marketplace using virtual currency, Creator Credits are a 1:1 equivalent to the marketplace in-game currency.