GraphQL is a query language for APIs that lets clients request exactly the data they need in a single request. The GraphQL contrib module provides a full GraphQL schema for Drupal content.
Unlike REST, where clients hit multiple endpoints and receive fixed response structures, GraphQL exposes a single endpoint and a schema that clients query with specific fields and relationships. This can be more efficient, especially for mobile apps on slow networks.
The Drupal GraphQL module has matured significantly over the past few years. It integrates with JSON:API, supports mutations for writing data, and respects permissions. For headless Drupal projects, it is an alternative to JSON:API that some front end teams prefer. The tradeoff is added complexity and a steeper learning curve. Most teams still start with JSON:API and only move to GraphQL if they have specific needs that justify it.