Calabash is a mobile app testing tool that uses Cucumber's Behavior-Driven Development (BDD) approach. It allows writing test steps in a natural language format (Gherkin) that both technical and non-technical team members can understand. The tests are run across both a local computer and connected mobile device, with Calabash libraries enabling the test code to programmatically interact with and control the app through gestures, assertions, and screenshots. Key components include feature files describing user stories to test, step definitions, the app being tested, and a test server installed on the device for communication between Calabash and the app.