For Plato, we have the "forms"(being) which is distinct from the world of "becoming" (physical sensible objects). i.e., a chair is a chair because it participates in Chairness.
For Aristotle, we have a physical object made of material substance. What is matter? Matter is the substance of physical objects. The way that the physical object is formed is what defines it as a chair. There is a form of "chairness" which is inherent in the physical object. This is different than Plato's transcendent form. Aristotle's is immanent.