Agnostic Theist? Isn't that an oxymoron?
Nope.
How does one believe in something yet not be sure it exists?
Because they have been told about it by others in a convincing manner, but have been given no evidence of its definite existence. For example, if you told me that you're wearing a red shirt and you have a banana on your desk, I would believe that you're wearing a red shirt and that there is a banana on your desk. I have no reason to disbelieve these things, because they are reasonable and believable things that happen every day, and I have no reason to believe that you are lying to me.
However, I don't know that these things are true for sure. I've not seen your shirt, I know that people can tell me things that aren't true, and I have no observable evidence pertaining to your fruit situation.
Thus, I would believe your statement yet be unsure of its truthfulness.
Another, even simpler, example: I believe that there is fresh cilantro in my fridge right now. However, I am not sure that there is. I bought some a few days ago, but I know my girlfriend made some delicious pico last night, so I'm not 100% sure that there's any left. I believe that there exists fresh cilantro in my fridge, yet I'm not sure that there exists fresh cilantro in my fridge.