A test case describes one situation to check and the result that should happen. It is like trying one specific step in a recipe and deciding in advance what the finished step should look like.
For a sign-in form, one test case might enter a valid email and password and check that the account page opens. Another case would use the wrong password and check that a helpful message appears.
Small, clear test cases make it easier to tell which behavior failed. They do not prove that every possible situation has been checked, but they help teams repeat important checks consistently.