The Graph is a decentralized protocol that helps with indexing and querying data from blockchains like Ethereum. It functions as a layer that sits between decentralized applications and their data sources, making the data easily accessible. Developers define how data should be indexed and structured from smart contracts using a subgraph manifest file. Graph nodes then continuously scan blockchains for new data and update the stored indices based on events from smart contracts. Decentralized applications can perform complex queries on the indexed data through the GraphQL interface.