The easiest way to understand Curve is to see it as an exchange. Its main goal is to let users and other decentralised protocols exchange stablecoins (DAI to USDC for example) through it with low fees and low slippage. Unlike exchanges out there that match a buyer and a seller, the behaviour of Curve is different, it uses liquidity pools like Uniswap. To achieve this, Curve needs liquidity (tokens) which is rewarded by those who provide it.