Supertest is a high-level abstraction for testing HTTP assertions, built upon Superagent. It simplifies end-to-end API testing in Node.js, providing a fluent interface for making HTTP requests and asserting on responses. Ideal for integration testing, Supertest reduces boilerplate code by handling request setup, execution, and response validation within a single, concise syntax. It's easy to integrate into existing testing frameworks like Jest or Mocha and ensures robust and reliable API endpoints.