It represents the fractal nature of the universe. As you zoom into a fractal you see that the fractal contains a form of itself. It also represents universes within unverses on the theory that blackholes contain other universes which in turn contain blackholes, and the concepts of "as above, so below" and "as in the macrocosm, so in the microcosm". And since this sequence is a dream, it represents a dream within a dream within a dream...etc. Thus is the dilemma of the main character in the movie. It is also reminiscent of neural nets. Each node in the fractal is another universe or a dream.