CFM=Cubic Feet Per Minute, that is airflow. For years most people equate suction with cleaning power, and while suction is important the TRUE cfm rating is much more important. Vacuum companies for years have shied away from this topic as very few vacuums have a high operating CFM. For instance a very popular canister the MIELE claims to have 140 cfm, WOW! but guess what that rating is taken before the motor is installed in the unit in an unrestricted environment, NO filters, NO bags, No 9 foot hose and wands, nothing. After restricted by the bag and filter and being 9 feet from the power source, The actual CFM of said unit as measured at the end of the hose is about 65, horrible for carpeting, just fine for hard floors. A simple rule of thumb is the further the motor is from the floor the worse the CFM rating will be. This is true on ANY VACUUM even the ones that truly do pull 140 cfm at the motor, will lose almost half of their power by attaching a 6 or 7 foot hose.
If you have a vacuum that has a true CFM rating of 65 and another that has a true CFM rating of 120. The vacuum with the higher Rating in this case would clean twice as fast.
Higher CFM will result in faster cleaning and overall better performance.