On the immediate level, myths are the stories we tell ourselves about that, which cannot be explained: Life and death, desire and fulfillment, emptiness, enlightenment and so on.
These stories can be interpreted in many ways, and can be said to mean many different things. But in the end, myth is irreducible. It cannot be summed up as-, or reduced to anything but what it is: A narrative of mystical origins. Stories.
There are always more things going on than what is apparent.
Take a lifestory, for example:
On the one hand I would say, that what I am writing now, the understanding and perspective that I am communicating, is caused by certain experiences, that allowed me to see things from a certain angel. These experiences unrolled in a certain order, and each played its important part in my forming of a worldview. And this worldview is the basis of what I am writing just now.
But on the other hand: What's a 'life'? There was no one specific thing that caused all of my experiences. Many different people, places, ideas, mind- and bodystates were involved, and each played their own part in making each experience exactly what it was. Without any one part, it would have been something else.
Yet I wouldn't say that my life could be summed up- or reduced to any one of the things, that made up my experiences. Each and every one thing was what it was, in order to make the whole exactly what it is - whatever the whole of life may be.
From the very same experiences, I could have told different story. And if I had arrieved at a different perspective, I would have told a different story. I would have been writing something else, communicating a different worldview.
I am not, though. For no matter how I look at it, I can't be anything but what I am. No matter what story I choose to tell. I could say it was something different, something else - but it wouldn't be. My experiences would still be my experiences, and my life would still be my life.
The same and different for everyone. Thus in conflict with the principle of non-contradiction. Therefore it cannot be the object of science. But it's still the most important thing there is, though it is vague and hard to grasp - like a dream.
A good book for those seeking to understand myth is Jungs Dreams, memories, reflections. An 'autobiography', but not in the traditional sense. It's a recollection of life as he experienced it. As stated in the foreword: It's him telling the myth of his life. The lifestory of a man, who was a mystery to himself.
As we all are, on one level or another.