Q:

What is res in res cogitans or res extensa?

Substance is that which has no dependent relation on any other; and unlike an atom, is infinitely differentiable - it has parts; and those parts thus distinguished have relations amongst themselves; and are subject to change (Lucretious on atoms: that which has no parts is not subject to change and is permanent).
Descarte introduced res cogitans and res extensa which is generally translated as  'thinking substance' and 'extended substance'; but it's in the notion of 'substance' by which dependence is not an possibility - thus from here, directly we have mind/body dualism; in the sense of being entirely distinct and without effect on each other.
This goes against what we observe 'clearly and distinctly': mind acting on body - I pick up a stone, and body acting on mind, I duck a stone flung in my direction.
Is substance then the correct translation for res? And what how did Descarte himself suggest their dependence? If he did. 
From item 52 from his Principles:

As for corporeal substance and mind (created thinking substance); they can be understood in terms of a single common concept, namely this one: things that don't depend on anything for their existence, except God.

This suggests that Descartes isn't conceiving substance in the classical form; the question is where he derives his notion of res in this sense; is he deriving it from earlier Scholasticism?

A:

The short answer is "yes". For Descartes the res is a substance on its own, and the term did derive from scholasticism. 
In the scholastic jargon, "res" (which literally just means "thing") was first used to be the name of any kind of entity whatsoever, be it a substance, an accident, a transcendental principle (such as unity, goodness, etc.). The term evolved in the context of a debate about the subject matter of metaphysics. What is metaphysics about? Aristotle says both that the subject matter of metaphysics is substance and that the subject matter of metaphysics is reality in general, being qua being. Briefly, the idea of the term "res" is that you can say that everything that is a res is part of the study of metaphysics in some way. Either because the thing you are talking about is a substance, or the principle of a substance, or an accident of a substance, etc.
In Descartes's usage though, it is clear that the res extensa and the res cogitans are both substances, less because of the use of the word "res" and more just because of the way Descartes describes both. The crucial fact is that Descartes thinks they can exist independently of each other. 

