Decentralized Web cannot feasibly use the same centralized service providers so prevalent in Web 2.0. This is why The Graph developers decided to build a data indexing protocol compatible with the data sources used in Web3: Ethereum blockchain, IPFS, etc. The Graph consists mainly of open APIs called subgraphs and has a variety of roles users can fill to help maintain the network for a reward. Monetary incentives such as stakes and fees are denominated in the GRT token. Node operators are called Indexers and as the name implies, their nodes index the data and process queries. Curators help to signal boost subgraphs, and Delegators share their stakes with Indexers for a share of their profits.
GRT is an Ethereum token of the ERC20 standard. There is going to be 10 billion GRT for everyone to use in this network, with about 6.9B in circulation at the time of writing.