Answer

This is a proof based on the assumption that God is the most perfect being we can conceive of. And if He does not exist, then there is a more perfect being we can conceive of—namely, a being that also exists. That contradicts the assumption that He is the most perfect being, and therefore it is clear that He exists.
If you want to discuss it, you’re welcome to read the first dialogue in my book The First Existent, where I dealt with this argument at much greater length.