The branch of philosophy that deals with that nature of reality.

Metaphysics is the field of study that tries to explain what exists. It tries to describe the vary basic nature of reality. Does the world have identity or is it chaotic and unpredictable? Is it deterministic or random? Do entities have a specific nature?

This branch of philosophy deals with "what is". It can be contrasted with how we come to know it. A scientific study may show us how a chemical reacts in a particular environment, for instance. But the nature of the chemical exists independent of our knowledge. Metaphysics deals with much more general ideas, but you can see through this example that the focus is not on how we're aware of it, but what is the nature independent of us.

A common usage of the term metaphysics describes the topic of mysticism, magic, ESP, and other topics that are non-scientific and unbelievable. That's not how it is used in the realm of philosophy.