In Ubuntu/Debian by default while creating ext2/ext3/ext4 file system 5% of disk space is reserved for super user across each partition.
Except for root partition 5% reserved space is not required for other partitions. So we reuse that reserved space.
Full article here:
Reduce amount reserved free disk space with tune2fs (Sany’s Linux and Open Source Blog)
tune2fs – adjust tunable filesystem parameters (Unix Tutorial)