SubQuery is an open-source, decentralized data indexer that provides flexible and fast GraphQL APIs for Web3 projects. It supports multiple blockchains including Polkadot, Ethereum, and Cosmos, allowing developers to extract, transform, and aggregate on-chain data by configuring a project manifest, defining a GraphQL schema, and writing mapping functions. Developers can run the indexer locally or deploy it to the SubQuery decentralized network, which is maintained by Indexers, Consumers, and Delegators, enabling full decentralization of data infrastructure.