A written description of one specific check: where you start, what you do step by step, and what result you should see.
A test case is the smallest unit of QA work. It has a title saying what is being checked, the starting conditions, numbered steps and a clear expected result. The idea is that any person, or any automated agent, can run it the same way and reach the same verdict: passed or failed. Without a written expected result, a test becomes an opinion.
Written cases turn 'we tried the site and it seems fine' into a list that shows exactly what was checked and what was not, and that can be run again after every change. In Fanann the QA Engineer writes the test cases after exploring your site, and you review them before they run, removing and adding as needed. For the basics, read what software testing is. Practical tip: write one case for the happy path, then one for every mistake a customer is likely to make: an empty field, an incomplete phone number, a declined card. Bugs hide in those cases, not in the happy path.