Descartes seems to define an idea as a representational mode of thought, a mode of thought which represents a certain object. For instance, the idea of the sun is a mode of thought which represents the sun, a certain 'object' in the world.
He goes on to give us a distinction between the objective reality and the formal reality of an idea. The objective reality is the reality of the representational content of the idea; every idea is the idea of something, it represents something. But the formal reality is the reality of the idea as it is itself something; every idea is something in itself.
The problem is clear. If an idea is by definition a representational mode of thought, then what does it mean to say that 'every idea is something in itself'? It is true--but isn't this 'something' a representation of an object? When we speak of an idea's formal reality, are we not therefore speaking of its objective reality, because there is no substantial distinction between the two. Of course an idea is a mental act, but in what way can this act be distinguished from its representational content?