The hidden month just another statistic that you may or may not care about. As Optimist on the run says in his answer, it is a tool to help you filter caches that will help you fill that table in your statistics. There is also one for hidden dates which does not factor the year in. The hidden month tool does take both year and month into account.
You should, however, select Optimist on the run as best answer. He was first and also correct :)