The Meditator reasons that all ideas are mere modes of thought, and in that sense they are all equal: they all have the same amount of formal reality, that is, reality intrinsic to themselves. However, what they represent differs greatly, and so their objective reality--the reality of the things they represent--also differs greatly. Thus, the idea of God has more objective reality than the idea of a tree, which has in turn more objective reality than the idea of the color red. Nonetheless, all three of these ideas are just ideas, and all have the same degree of formal reality. (The commentary section below will explain in more detail what is meant by "formal" and "objective" reality and what it means to have more or less reality.)
The Meditator asserts that no effect can have a greater amount of reality than its cause. That is, everything that comes into being must be made to be by something that has an equal or greater amount of reality. For instance, a stone can be made by chipping off a larger piece of rock, since the larger rock has more reality, but a stone cannot be made out of a color, since a stone has more reality than a color. The Meditator also suggests that an idea can only be caused by something that has as much formal reality as the idea has objective reality. The idea of a stone, then, could be caused by a stone or a large rock but it could not be caused by a color. The Meditator grants that ideas can be caused by other ideas, but that there must ultimately be something more than an idea that is the cause of these ideas. The first cause of an idea must be something with at least as much formal reality as the idea has objective reality.
If he can conceive of some idea with so much objective reality that it must come from some cause with more formal reality than he herself possesses, the Meditator reasons that he will then know that something outside his mind exists. His ideas of other people, animals, and angels can easily come from himself even if no such things exist. Similarly, corporeal things contain nothing so great that it could not originate in him. From the reasoning of the Wax Argument, he has concluded that he can only clearly and distinctly perceive properties like size, extension, shape, motion, duration, number, and substance when examining corporeal things. Most of these properties the Meditator himself also possesses. And even if, as a thinking thing, he may not have size, extension, shape, or motion, these properties are modes of the substance of body, and, as a thinking thing, he is a substance, and therefore has more reality than these modes. (Cartesian ontology, modes, and substances, will also be elucidated in the commentary.)
Sensible qualities like color, sound, smell, taste, heat, cold, and so on are only perceived in a confused and obscure way, and the Meditator is not even certain as to whether or not they are things or non-things. If they are things, they must have such a small degree of reality as to originate unproblematically from the Meditator himself.
This section of text dives right into a number of distinctions made by the medieval Scholastic philosophers that would have been very current in Descartes' day. Their currency has since waned and these terms are no longer familiar to the ordinary reader, so what follows will be a brief tutorial on Cartesian ontology and distinctions within the theory of ideas.
For Descartes, as for most thinkers of his time, the fundamental building blocks of reality are called substances. Substances can exist independently and are indestructible. In Cartesian ontology, there are two kinds of substances: bodies and minds. Of substances that are minds, there are finite minds, such as people and angels, and there is the infinite mind, God. Thus, when the Meditator asserts the sum res cogitans, he is asserting that, as a substance, he is a mind rather than a body.