A DeFi aggregator is a platform that aggregates decentralized finance (DeFi) services, allowing users to compare and access the best rates for lending, borrowing, and swapping assets. These platforms gather liquidity from multiple DeFi protocols, providing a seamless user experience. DeFi aggregators optimize yields by identifying the best opportunities across various decentralized exchanges (DEXs) and lending platforms, saving users time and effort. Popular examples include Yearn.finance and 1inch, which maximize returns by automating and simplifying the DeFi process.