Res extensa is one of the two substances described by René Descartes in his Cartesian ontology (often referred to as "radical dualism"), alongside res cogitans. Translated from Latin, "res extensa" means "extended thing" while the latter is described as "a thinking and unextended thing". Descartes often translated res extensa as "corporeal substance" but it is something that only God can create.
Res extensa vs. res cogitans
Res extensa and res cogitans are mutually exclusive and this makes it possible to conceptualize the complete intellectual independence from the body. On the other hand, res extensa, are entities described by the principles of logic and are considered in terms of definiteness. Due to the polarity of these two concepts, the natural science focused on res extensa. The categorical separation of these two, however, caused a problem, which can be demonstrated in this question: How can a wish (a mental event), cause an arm movement (a physical event)? Descartes has not provided any answer to this but Gottfried Leibniz proposed that it can be addressed by endowing each geometrical point in the res extensa with mind.