Just what in heck is an "agnostic"?

Let’s face it. Either there is evidence enough to “believe” something, or the evidence is not available, or credible, so you do not “believe” it. It is certainly OK to say “I don’t know”, but how long are you going to put off committing? I have friends who have been “agnostic” for 20 YEARS for heaven’s sake. How can you be uncertain for 20 years?

So maybe the problem is that I do not really understand what an “agnostic” really means when they say “I am agnostic”

So, enlighten me.