End-To-End (E2E) testing is all about testing your application against specific scenarios. From a requirements-engineering-perspective you'd call those User Stories. Essentially, these stories describe a series of actions a user performs to achieve a certain goal. So by assembling a bunch of these - referred to as a test suite - you are able to verify that your web app acts as expected. The key to success, of course, lies in the amount and quality of tests written.
With E2E testing you are not interacting with the app's code per se, but with the app's interface. This is different than unit tests, which take care of isolated parts of the application - called units - by verifying them through the removal or mocking of other parts. It's important to note that one method of testing does not replace the other, so don't take this article as an excuse to skip unit testing.
