In the sentence, "Socrates is a human", the subject refers to an individual and the predicate refers to the universal "humanity", over and above any individual human. "Humanity" is an abstract; it doesn't refer to any concrete individual but rather to the concept of "being human." For Aquinas, universals are not just in our minds but are quite real. Unlike Plato's famous theory of Forms, however, he doesn't think they exist in a third ghostly realm. He thinks they just exist in the things themselves. The humanity of Socrates exists in Socrates himself.
II. Potentiality and Actuality
Things change: rivers flow, trees grow, people are born and die. Change in involves the switch from potential to actual. A seed is actually a seed and potentially a tree. When it becomes a tree, the potentiality becomes actuality. Most things around us are a mix of potential and actual. Something cannot be just potential with no actuality, as this is a logical contradiction. E.g., you can't have a potential tree that is not also an actual seed. However, there is no contradiction in the idea of something that is just actual.
III. Matter and Form
The normal objects of our experience consist of a composite of form and matter. They can't be broken down any more than that composite. So for example, Rex would consist of matter + the form of dog. There is no such thing as "just matter" with no form, because even a single quark has the form of: one quark.
IV. Essence and Existence
The "essence" of something is that list of properties that make it the thing it is rather than some other thing. The essence of a triangle, for example, is to have three sides (among others). If it didn't have that, it just wouldn't be a triangle. "Existence" is whether the thing actually exists or not. For most things in our experience, existence and essence are separate. I.e., existence is not included in the essence of most things.
V. Four Causes
To get a complete description of something, you need to know four aspects of it: what it's made of (material cause), the structure it takes (formal cause), what brought it into existence (efficient cause), and what it does (final cause).
VI. Potential and Actual Applies Across the Board
The potential/actual distinction applies across the entire Thomistic metaphysical system. Matter is potential, and when conjoined with form it becomes actual. Essence is potential, and when conjoined with existence it is actual.