In ancient Greek the word meant 'beyond physics'. This has often been taken to mean 'beyond the physical' ie concerned with the spiritual or supernatural, but in fact originally the term probably referred to some writings of Aristotle that came after his books on physics. Still, the idea of metaphysics as being connected with 'things beyond the physical' has persisted.
In a very broad way, metaphysics is a branch of philosophy that is concerned with the nature of being. Aristotle divided metaphysics into the study of being, including the classification of things, the study of gods, eg the nature of the divine, and the study of first principles - the foundations of philosophy, science, logic etc.
Nowadays we tend to use the term rather vaguely, to refer to general spiritual ideas. The metaphysical poets of the 17th century were later so called because they explored abstract and obscure ideas in their work.
You can find a good overview of the origin and ideas of metaphysics here.