End-to-end testing, or E2E testing, as the name suggests, is the testing methodology that validates a software workflow from start to end. The primary goal of E2E testing is to simulate an end user's application journey and ensure that all integrated components, dependencies, and other integrated pieces work as expected.
While this looks obvious that the testing has to be done with all components attached, end-to-end testing is the last stage of testing and is usually done infrequently. This is especially so if the application under test is extensive, i.e., it has hundreds of components and thousands of dependencies.