From the GraphQL site

GraphQL is a query language for your API, and a server-side runtime for executing queries using a type system you define for your data. The GraphQL specification was open-sourced in 2015 and has since been implemented in a variety of programming languages. GraphQL isn’t tied to any specific database or storage engine—it is backed by your existing code and data.

zkool_graphql is a GraphQL server for the Zkool wallet engine. It offers a typesafe API to wallet creation, synchronization, address generation and payments.

It can be used to develop apps that leverage the Zcash privacy technologies.

Unlike the classic JSON RPC, GraphQL is object oriented, supports custom queries and push notifications.

Queries

Queries are requests that return information about your wallets without making any modification. Unless a query refers to the blockchain (which changes externally), calling it multiple times returns the same result.

Mutations

Mutations are request that may change the data of the wallet. For instance, creating a new account, making a payment, or synchronization are mutations.

Subscriptions

Subscriptions are long running queries that lets the server send notifications to the client. The app can subscribe to events for new blocks and incoming transactions.