[Author's note: This post is about the philosophical concept of metaphysics. If you want to learn about the more common usage of the word, try looking it up using Google. This started out as a comment to a previous post but I decided that a full post should be used to explain this concept before I finish my comment.]
Metaphysics is perhaps one of the most misunderstood and misused ideas in our modern world (especially when it gets applied to religion and then it becomes a misunderstanding squared, a L^2 error if you will). So here is a brief run down of metaphysics (this is important because meta-_____ and metaphysics keep popping up in places where you least expect it, even if it is not mentioned by name).
The word metaphysics originally came from the Greek μετά (meta) and φυσικά (physics) (literally "after" or "beyond" physics). The term came about because Aristotle wrote several books and one of them was named Physics and another one was named First Philosophy. Several years later when Aristotle's standard works were compiled into a single collection, the main editor of the collection just happened to stick the book First Philosophy after the book Physics, much in the same way the book Zephaniah follows the book Habakkuk in the Bible (i.e. there may have been a reason that made sense to person who stuck it there, but in the end it's just the way the cookie crumbled). After a while the book First Philosophy was referred to by Greek speaking scholars as being meta-physics, meaning, "That book after the book on physics" (or to put it in Greek: "τὰ μετὰ τὰ φυσικά").
When Latin speaking scholars heard about this they misinterpreted the nickname for the book to hold some special insight and meaning for what the book contained and thus the name meta-physics (again, literally "after" or "beyond" physics) was used by Latin (non-Greek) speaking scholars to signify anything that went beyond the normal study of physics, or anything that looked into why physics was the way it was (eventually it got associated with a lot of spirituality, occult and Eastern Religion stuff, which is why modern metaphysicists will never admit that that is what they do. They will always insist that they do something respectable called "theoretical physics" and not that dirty pseudo-mumbo-jumbo, but that is a different story).
So, now that the history lesson is out of the way, the term "metaphysics" does have a precise technical meaning that has nothing to do with the original meaning. So now me may ask how do philosophers use the term metaphysics today, and what does it mean to be "meta"? To explain this I will use a simple analogy. Think of a house. Think of all the stuff inside. The sofa, the TV, the beds, the chairs, the tables, the fridge (complete with moldy and rotting left-overs that you haven't eaten yet...yeah, you should probably throw those out. I'll wait.). In this imaginary house, all the things you interact with is analogous to physics (and all its related fields of chemistry, biology, geology, astronomy etc.). The things you bump up against on a daily basis are the things that make life possible and determine things like weather, baseball games, carney rides, postage stamps and glow worms.
But in our imaginary house you find that you can't put stuff wherever you want. You have to deal with things like walls, electrical outlets, light fixtures, water lines and door frames. It doesn't matter if you really, really, really, really want to put your washing machine in the middle of your living room. You can't. There's no water hook-up and no power outlet. So you find that in some cases your physics is constrained by something else. Something that you don't (and can't) interact with on a daily basis. The frame of the house, with all of the electrical wiring and pipes, is the thing that determines the physics of your daily life. The part of the house that you do not always see, but determines whether or not you can buy that 2000" TV, is the metaphysics.
So metaphysics is the thing that fundamentally determines everything else about reality (and in a very colloquial sense, to "go meta" or to "be meta" means to go to the next level of what determines our reality). For modern physicists who hear the word "metaphysics" and think of people rubbing holy oils all over their bodies and chanting strange chants while connecting to the spirals of the universe through the crystals, it may come as a shock that theoretical physicists are nothing more than metaphysicists with a degree in math. In the philosophical sense theoretical physics, including quantum mechanics, general relativity, string theory, cosmology and everything in between is just the modern version of Aristotle's metaphysics. They are trying to get to and understand the fundamental structure of reality and find out what determines everything else. And that is precisely what metaphysics is.
Returning to my house analogy, if you read through it and start thinking, "What about the stuff that makes up the framework? The wood and metal and other things determine the framework of the house just like the framework determines the house." Well, as Dr. Chris Foster would say, "Congratulations you just made a meta-analogy! I'm about to go meta on your meta! You meta?" So if you get that, then you get the concept of metaphysics.