A test filled in a form and submitted it. Green. The same steps by hand showed a validation error the test never saw.
The test had set the field's value directly instead of typing into it. The app was listening for typing. No typing, no validation, no error. The test was doing something a user cannot do.
That is why this lecture exists. Your test pretends to be a person, and the pretence has to be close enough.
A click is not one event. When a user clicks a button, the browser produces a small sequence: the pointer moves over the element, the button goes down, the button comes up, then the click. Apps listen at different points. A menu might open on hover, before any click happens.
Frameworks handle this for you, but they also check things first. Most refuse to click an element that is invisible, disabled, or covered by something else, because a user could not click it either. When your test fails with "element is not clickable", that is usually correct and useful. Something is covering your button, and a real user would have the same problem.
Typing is a sequence too. Each character produces its own events, and apps react to them: search boxes filter as you type, forms validate as you leave a field, save buttons become enabled once a field is not empty.
This is exactly what went wrong in the story above. Setting a value in one step skips all of it.
Navigation is the user moving between pages, by clicking a link, submitting a form, or pressing back. In a modern app, most of these do not reload the page at all. JavaScript swaps the content and updates the address bar. This matters to your test, because "the page changed" and "the page reloaded" are different, and one of them wipes state while the other does not.
Forms are where most real testing happens, so learn the shape of them. A form has fields, rules about what is allowed, and a submit action. The interesting part is almost never the happy path. It is what the form refuses: an empty required field, a badly formed email, a password that is too short, a card number that fails its check.
Here is the habit that separates a useful tester from a beginner. When you meet any form, ask what it should reject, not just what it should accept. A login test that only tries the correct password would pass on a login that lets absolutely anyone in. That is not a hypothetical; it is one of the first problems you will solve here.
Remember this: your test should do what a user does, in the order a user does it. When the framework refuses an action, it is usually telling you something true about the app.
Reference: MDN, introduction to events.