Hidden test cases include your interviewer's corner cases or different scenarios defined to validate your coding solution. These test cases check whether your solution addresses the problem, including its various constraints, but do not display the expected output of the test case.
For instance, the hidden test cases may be defined to validate your coding logic against boundary values, error handling scenarios, etc. Depending on the complexity of the coding question or for specific skills assessment, your interviewer may include one or more hidden test cases for your question.
To know how hidden test cases work, refer to the Hidden Test Cases article.