Imbalance is a condition when a rotating part’s mass does not lie at the center of rotation. This causes a force (vibration) at the frequency of rotation. If the vibration is significant, it can shorten the life of the bearings and damage other parts of the spindle.
Imbalance can be corrected, weight can be attached in various ways or material can be removed by grinding or drilling. This moves the center of mass back to the center of rotation. This process will improve spindle performance and add to the longevity of the spindle bearings.