The Fuel GraphQL API allows you to query the Fuel blockchain for a wide range of on-chain data. It can be used to query transactions, balances, block information, and more. You can also use it to simulate and submit transactions on the Fuel network.
The playground is an interactive and graphical IDE that includes a reference for queries, mutations, and types. It also provides query validation and context for the underlying GraphQL schema.
You can test out the Fuel GraphQL API playground here:
https://testnet.fuel.network/v1/playground
This is the RPC endpoint you can use in your applications to interact with the Fuel blockchain, whether it's retrieving on-chain data or sending transactions:
https://testnet.fuel.network/v1/graphql.
Note that this is the same endpoint used in the playground above.