Creatures have a symbiotic relationship with Midi-Chlorians in their bodies--microscopic creatures that reside in a living being's cells--and those creatures can manipulate each other. A Jedi or Sith can learn how to basically control what their own Midi-Chlorians do to the world around them.
This is a very common misconception. Midi-chlorians are NOT the Force, nor are they the cause of it. The Force is almost like some kind of omnipotent deity, a spiritual energy that seems to have a will of its own. Midi-chlorians are only an indicator of one's potential with the Force.