What is validationQuery for database and JNDI?

I seem to have the exact same issue as the poster of this original SO question .

The problem is that he answered his question with the following comment:

I solved my problem, I added validationQuery to my JNDI data source.

Unfortunately, this does not mean anything to me and does not help me solve my problem. So I ask: what is the " validationQuery" in relation to my database (Sybase) and the JNDI record used to store server search information?

Thanks in advance.