A DeFi aggregator is a platform that consolidates various decentralized finance services, enabling users to access multiple protocols and yield farming opportunities through a single interface. It streamlines user experiences by optimizing trades, reducing transaction fees, and offering the best rates across different DeFi platforms. Aggregators like 1inch and Yearn Finance enhance liquidity and improve the efficiency of DeFi operations by sourcing liquidity from multiple decentralized exchanges (DEXs) or lending protocols. They ensure users can make the most profitable and efficient financial decisions in the DeFi space.