In short Decentralized Finance — lending, trading and yield products built from smart contracts, open to anyone with a wallet, no bank account required.
Because the pieces are public code, they snap together: a loan taken on one protocol can be traded on a second and used as collateral on a third, in a single transaction. That composability is where the yields come from — and where the risk comes from, since a fault in one layer propagates to everything stacked on it. Yield in DeFi is never free money; it is payment for lending, for taking price risk, or for accepting that a contract might fail.