Big questions.  I'm going to define some things first, so forgive me if I state things you already know.

Metaphysics, broken down means "above" physics. Meta - 'above.' Historically, it has meant concepts, ideas or non-physical phenomena: things that exist but have no discernible substance or matter. Souls, thoughts, and abstractions and qualities are common examples. 

Epistemology is the study of knowledge: what we know. How we know 'what we know' is a matter of debate. Do we know things only through experience (empiricism) or can we know things just using reason and though (rationality), or are there other ways of knowing things: intuition, a priori knowledge, a memory of absolute Forms (Plato).

Ontology is the science of being: what a thing (or event) actually is.  To differentiate ontology and epistemology for the sake of contextualizing all of this, think of it this way:

Kant's terms:

phenomena - how we perceive reality through experience, including reason or thought.  we see reality with a kind of lens - this is a cliche but an effective way of describing the perspective of experience.

noumena - things as they essentially are.

So, epistemology is the study of how we see the world - phenomena - (Reality) and ontology is the attempt to understand being, how a thing actually is - noumena -(Actuality). 

Let's say you see a building in the distance. The phenomenal existence, to you, is that it is a building. How do you know it is actually a building? Well, from experience of seeing other buildings and reasoning that it must be a building because it looks like on, it's adjacent to other buildings in a city, etc. But you walk to it. When you reach the "building" you discover it is a hologram. From a distance, you couldn't conclude it was a hologram. You had to "get to it."

A worldview, in this context, is how you get to the allegorical hologram. I use "allegorical" because it could be anything. The worldview in philosophy with respect to epistemology and ontology is your method or (teleological) approach to understanding things as they are via experience or beyond experience. Some think mathematics is the underlying noumena of all phenomena (an equation gives you a SIN wave). Some think it involves a structured way of thinking. Different worldviews apply to different methodologies of reality (or Actuality, to stick with the terms we are using). Marx used historical materialism to "get to" the underlying mechanism of historical development. Heisenberg used quantum mechanics. Kant developed a system of understanding most things but believed there were things that escaped human capability.

I am dodging your last question. How important is it? Extremely important: depending on its applications. Marx's philosophy was a response to oppressive economic forces, so there is an ethical and humanistic impetus there. Other worldviews have included theological, social, and ethical applications. In the broader sense, I think most philosophers today would say a worldview includes much more: feminism, religion, science; ecological sustainability is a worldview based in science with social applications.

Knowing when something is a hologram and when it is a building is the process from epistemology to ontology. That process is usually embarked upon with other concerns in mind: the ethical, social, technological etc. the understanding is the icing on the cake: if there is a cake. Philsophy is the process.