The Hasura GraphQL Engine is an open-source product that provides instant realtime GraphQL APIs on Postgres. It allows developers to add GraphQL APIs to existing Postgres databases and includes a UI for creating and viewing tables, as well as making GraphQL queries. The Hasura GraphQL Engine supports subscriptions for all kinds of queries and is designed to be lightweight and high-performance. Companies with existing Postgres applications can incrementally migrate to GraphQL without affecting their current setup.