will give a very good and clear description of the dreamtime. As Amergin said, it's a combination of cultural heritage, spirituality and explanations of how the world came into being. Lovely philosophy which ties people to the land and to the whole creation. Like all socio-religious belief structures, metaphor and real life are closely intermixed.
For a real understanding of the dreamtime, speak to an original aussie, if there are any on the 'Cat.