"Agnostic" means "not knowing." It was coined by the biologist Thomas Huxley in the 1800s to express the idea of not knowing whether or not there is a God (as opposed to atheism, the belief that there is no God).
In software development circles, people see an analogy to religion in the intensity and fervor with which some developers advocate for their favorite computer languages, platforms, hardware, and in this case DBMS's.
To be "agnostic" means you don't have a strong belief that any of them is the one true DBMS and that you are willing to support all of them.