A DeFi aggregator is a platform that consolidates multiple decentralized finance services into a single interface, enabling users to access liquidity, lending, borrowing, and yield farming from various DeFi protocols. Aggregators help users find the best rates, improve returns on investments, and reduce the complexity of managing multiple DeFi platforms. They are crucial in simplifying the DeFi ecosystem, offering efficiency and maximizing user profits by optimizing access to decentralized financial products.