The ontological argument is considered unique amongst arguments for God’s existence because it is the only argument that can be classified as an a priori argument. By this we mean the sequence of logic we follow to draw the conclusion is not dependent on experience and the conclusion, if the premises are correct, must necessarily be so. The strength of the ontological argument rests on the idea that existence is a property or attribute of a concept, sometimes called a “predicate”, much as say colour, length or mass would be for an object. So, for example, the “most real” $100 bill must exist – for, unless it exists in actuality, I cannot spend it. Thus, if God is conceived of as the “most real” being, God must exist.
Kant’s line of attack, which this essay attempts to explicate, is to show that existence is not a predicate but something which is added later to a concept during its actualisation. He asserts that statements about existence are synthetic – they can only be validated by experience (in this case – does it exist? I have to look to confirm that), they are not propositions of logic or necessarily included in the definition (known as analytic statements, e.g. a triangle is a three-sided figure; to be a triangle includes the predicate, ‘3-sided’).
You can find the essay here.