Every movement is to be done with control, so you aren't just throwing your body around. Third is centering, so that you are evenly using your body. Think of a plumb line down the middle of the body working both sides evenly. The fluidity of the exercise is fourth. It's important because once you've learned the routine, it should look something like a dance where every movement flows into the next.
Precision is fifth.
You try to make each movement as precise as possible. Finally, breath is also very important to the exercises. You don't want to hold your breath at all. So again, concentration, control, centering, fluidity, precision and breath are the six basic principles. Describe one of the exercise in Pilates.