Something that is fluted is grooved...think of architectural columns with grooves around the periphery.
In dentistry, you often see fluting (vertical grooves/depressions) on tooth roots. For example, the distal aspect of the mesial root of mandibular molars. This area has a fairly prominent groove and can therefore be dangerous during endo...ie. perforation of the root. Another very common area of fluting is the mesial aspect of maxillary first premolars.