Metaphysics - the study of 'reality', or 'how the world works', or the like. Originally, one must recall, there were no branches of science - instead, it was all philosophy. The concept of 'Astrophysics' was considered 'Metaphysics' to philosophers like Copernicus. Over time, as 'philosophers (i.e. theoretical scientists) became more specialized, disciplines would separate out. Psychology didn't become it's own discipline until the late 1800s).
So metaphysics is the branch of philosophy that deals with what we know, or can know, about how the world works - i.e. theoretical science - with, or without, the scientific process.
It takes it further, however, and discusses what we, as humans, can EVER know about the machinery of the universe - or if such machinery is an illusion. Many of these arguments aren't really presented 'seriously', but are encountered as a natural extension of an earlier argument. Essentially, it comes down to using Logic (the philosophical argument) to break each question into its base parts.
If A, then B. If B, then C. Therefore, if A, then C.
One way to think about this is that even in the modern world, we do not have many scientific LAWS. We have theories, and many of these theories are consistenly proven to be useful, but we don't know if they're true. Philosophy discusses what things MUST be true, and which things MAY be true.
It's as if we're looking at the universe as if it's a giant clock. We can postulate how the insides work, as we can see the hands move, and hear the ticks - and we can predict things like the tick preceeding a movement of a clock hand - but we can't see inside the clock. When we get to the small nature of things, like quantum physics, we're still guessing - every generation finds smaller 'building blocks' - atoms, quarks, etc.
So can we 'know' metaphysics? Not sure. But saying that we cannot truly know metaphysics (i.e. how reality, or science, works), is in itself a metaphysical statement - as it's a statement about how the world works (i.e. saying we can't know, is a statement about how much we know).
That being said, that 'cliff's notes' version is certainly not all that metaphysics is, or discusses - it's a very narrow view about one aspect of metaphysics, and it wouldn't even be looked at until a third year philosophy course, I wouldn't think.
Skip those heady discussions (which presuppose that you've studied people like Aristotle, Plato, Descartes), and instead read Aristotle's Diomachian Ethics, Plato's Republic, Descartes, etc). Or better yet, start with the really early Greek philosophers like Anaximander of Rhodes, Demosthenes, etc. Those are more fun, and develop the building blocks to discuss this sort of nitpicky issue.