A DeFi aggregator is a platform that consolidates various decentralized finance protocols and services into a single interface, allowing users to access the best possible yields, liquidity, and investment options. These platforms help streamline processes like lending, borrowing, and trading by sourcing opportunities from multiple DeFi platforms. They optimize returns for users by automatically comparing rates and executing transactions at the most favorable conditions. Popular examples include 1inch and Yearn.finance, simplifying DeFi participation and increasing overall efficiency.