This text discusses how GraphQL can be used to create a unified API layer for querying disparate data sources, such as PostgreSQL and Amazon DynamoDB. It explains the concept of a supergraph, which represents a unified view of an organization's entire data graph, allowing seamless integration and querying of disparate data sources through a single API. The text then demonstrates how to connect Hasura to a PostgreSQL database to create a subgraph, how AWS AppSync connects to Amazon DynamoDB to create another subgraph, and finally how Hasura can federate these APIs into a supergraph, establishing a relationship between PostgreSQL and Amazon DynamoDB data. The example provided is an e-commerce use case where the goal is to query product data from PostgreSQL and inventory data from DynamoDB in one request using GraphQL.