FlexVol volumes have a maximum number of files that they can contain. Knowing how many files are contained by your volumes helps you determine whether you need to increase the number of (public) inodes for your volumes to prevent them from hitting their maximum file limit.
Public inodes can be either free (they are not associated with a file) or used (they point to a file). The number of free inodes for a volume is the total number of inodes for the volume minus the number of used inodes (the number of files).
shl5> df -i Filesystem iused ifree %iused Mounted on /vol/root_vs0/ 97 19893 0% /vol/root_vs0/ /vol/vol0/ 8455 10977711 0% /vol/vol0/ /vol/v1/ 100 1096 8% /vol/v1/