A fret dress is a process of fileing down the frets insuring to be perfectly level and then re-crowning.
This is needed when an instrument has uneven frets causing fret buzz and not allowing the action to be set at the required height. A fret dress can 9 times out of 10 be a cheaper option than a re-fret for sorting out excessive fret wear and can also solve some minor neck defects.
Instuments that need a fret dress is very common even on high end guitars/basses especially if you require a low action,
this is because when instuments are manufactured the
frets are pressed in with a fret press and are never perfectly
leveled afterwards.
Please check out me doing a fret dress in the video below