Test cases help in validating candidates' code. A test case consists of an input to the code and an expected output. Once candidates submit the code, it is run against all the test cases. The output from the candidate’s code is compared with the expected output to see whether the test case has passed or failed.
So, a test case is successful when your program produces expected output for that particular input. It is a failure when your program does not produce the expected output (either the output is wrong or the code execution failed because it took too long to respond). A large test case means a test case with a large input.