It means they are evident on a purely natural level, i.e., without the supernatural helps of grace, faith, revelation, etc. St. Thomas Aquinas says natural law is something pertaining to reason: “the natural law is something appointed by reason” (What is the natural law?).
Or the principle of non-contradiction, which St. Thomas Aquinas formulates as (, I-II q. 94 a. 2Summa Theol. c.):
Wherefore the first indemonstrable [For why, see the quote below from St. Thomas’s commentary on Aristotle’s *Metaphysics.]
principle is that “the same thing cannot be affirmed and denied at the same time,” ** which is based on the notion of “being” and “not-being”: and on this principle all others are based, as is stated in Metaph. iv, text. 9. Now as “being” is the first thing that falls under the apprehension ** simply, so “good” is the first thing that falls under the apprehension of the practical reason, which is directed to action: since every agent acts for an end under the aspect of good. Consequently the first principle of practical reason is one founded on the notion of good, viz. that “good is that which all things seek after.” Hence this is the first precept of law, that “good is to be done and pursued, and evil is to be avoided.” All other precepts of the natural law are based upon this: so that whatever the practical reason naturally apprehends as man’s good (or evil) belongs to the precepts of the natural law as something to be done or avoided.
Are you asking why “good is that which all things seek after” rather than evil, which things can indeed seek after to a certain degree? The reason is simple: things seeking evil tend to lose being, and in so doing they lessen their being a thing. If a being seeks evil completely, it ceases to exist. So all things cannot seek evil because then they would be non-things, and this would violate the principle of non-contradiction.
So, why is the principle of non-contradiction indemonstrable? Commenting on Aristotle’s Metaphysics, St. Thomas writes (Commentary on Aristotle’s Metaphysics, book 4,lecture 6 [607.]):
[Aristotle] says, first, that certain men deem it fitting, i.e., they wish, to demonstrate this principle; and they do this “through want of education,” i.e., through lack of learning or instruction. For there is want of education when a man does not know what to seek demonstration for and what not to; for not all things can be demonstrated. For if all things were demonstrable, then, since a thing is not demonstrated through itself but through something else, demonstrations would either be circular (although this cannot be true, because then the same thing would be both better known and less well known, as is clear in Book I of the Posterior Analytics
[cf. St. Thomas’s lecture: [/COLOR]Expositio Posteriorum, lib. 1 l. 7]), or they would have to proceed to infinity. But if there were an infinite regress in demonstrations, demonstration would be impossible, because the conclusion of any demonstration is made certain by reducing it to the first principle of demonstration. But this would not be the case if demonstration proceeded to infinity in an upward direction. It is clear, then, that not all things are demonstrable. And if some things are not demonstrable, these men cannot say that any principle is more indemonstrable than the above-mentioned one.