Dust motes are those small particles (the larger ones look like thin fibers, the smaller ones look like...err...motes) which you may observe in a shaft of sunlight in an indoor or forested environment.
An Interesting (and Little Known) Fact: Many Pre-Socratic philosophers believed that your soul was made up of dust motes (Democritus and the Pythagoreans). Wacky, true, but their reasoning is sound; they thought that the soul must be the source of movement---and dust motes were the only things which they observed moving without an apparent mover ("these motes were referred to because they are seen always in movement, even in complete calm"). They concluded, then, that dust motes have an internal principle of motion, and that your soul (internal principle of motion of the body) must be made out of dust motes.
See Book I, Chapter 2 of Aristotle's De Anima.