Explain the ontological argument

Our concept of God is that of a being greater than which none can be imagined. Since we are able to imagine God at all, then we can see the God exists as an idea in the mind. However, it seems that a thing which exists in the mind and in reality is necessarily better than a thing which only exists in the mind. Since this is the case, if God were to only exist as an idea in the mind, then we would be able to imagine a greater being. However, we are unable to imagine a greater being, and therefore God must exist.