An end-to-end test checks whether a complete task works from the user's point of view. It is like following a whole journey from the front door to the destination, rather than checking one road sign.
For a website, a test might open the sign-in page, enter details, submit the form, and confirm that the account page appears. It exercises the visible steps and the connections behind them.
These tests can catch problems that small isolated tests miss, but they are usually slower and can fail when the surrounding test environment is unreliable.