This is like trying to explain quantum physics to a four year old. If you want me to make it easy for you, I'll use an analogy. It's a fucking stupid analogy, but it serves my purpose.

Have you seen The Matrix? I imagine you have, so follow me here. Just because you can touch the things in front of you, or eat the food in your refigerator that doesn't make those objects real. Everything you experience in everyday life could be nothing more than a dream. Much like The Matrix...(God, I hate using this ridiculous analogy).

The point is this. Just becuase you say you exist, doesn't mean you exist. Just because you believe you are really here, doesn't actually mean you are really here. It could all be in your head. That's where faith comes in. If you don't believe in God because there is no "proof", then technically you can't believe in anything. Including your own existence.