It's a simulation of a world, where there is plants (green circles) and creatures (colored squares). First there is just one big plant and one creature. The creature has abilities to multiply, mutate, eat, move, etc., and through random mutations those abilities may change over time.

The first mentioned creature is small omnivore (can eat both plants and carcasses of dead creatures), which has very poor ability to move, actually it can move only randomly without any sense of direction. However, it's quite fast to multiply and extremely sensitive to mutations. Maybe, with a little luck, one of it's descendants gains the ability to move to the next food source once the original "eden" is consumed