In case you are completely new to GraphQL, I will recommend you to start with our Introduction to GraphQL or GraphQL Core Concepts
A schema is the core component of a GraphQL application. It defines the capabilities of the GraphQL application. To elaborate further, a GraphQL schema is like a description of the data available from your application. It also defines the various queries and mutations that clients can use to interact with the server. In a way, the schema is the heart of a GraphQL application.