Animism is the doctrine that every natural thing in the universe has a soul. If you believe in animism, you believe that ostriches, cactuses, mountains, and thunder are all spiritual beings.
Animism comes from the Latin word anima, meaning life, or soul. People often think of "primitive" beliefs when they think of animism, but you'll find the belief in the spiritual life of natural objects in all major religions. If you have ever felt that a star is more profound than a cluster of hot rock or gas a long way away, then you have a sense of what animism is.