The natural light, and its corresponding effect, sc. clear and distinct ideas are central to the argument of the Mediations, but Descartes leaves the ideas relatively undeveloped.
Descartes opposes the natural light to ideas that arise in him by a spontaneous impulse, and it seems to me that he has more than one thing in mind, viz:
1.) Claims whose only warrant is repeated experience.This swan is white, that swan is white, that other swan is white… if we see things constantly together, there is a spontaneous impulse to think that they must go together.
2.) Things that are associated by a more or less superficial resemblance. Wild rice looks like rice, whales look like fish, unjust laws do lawlike things, “Cancer” gives one the impression that it is a single disease that could be unlocked by a single cure, etc.
3.) Things that are divided by more or less superficial differences. Cannonballs and the moon, corn and the grass in my yard, etc.
Things known in the natural light, in opposition to these, appear to be known by the mind perceiving a necessary nexus between the subject and the predicate in a way that does not reduce to experience or appearance. Even if these ideas require an experience, the warrant for the their truth is not repeated experience. Descartes gives the examples of the greater angle in a triangle subtends the greater side, the less perfect cannot formally cause the more perfect, and God exists.