Yield Protocol is an Ethereum-based protocol for collateralized fixed-rate, fixed-term borrowing and lending.To achieve its goals, Yield uses a class of tokens called fyTokens (fixed yield tokens). fyTokens are Ethereum based ERC-20 tokens that can be redeemed for an underlying asset one-to-one after a predetermined maturity date. For example, if you have one fyDai token, you can redeem it for one Dai after the maturity date.