A fret end dressing is required when the ends of the frets are uncomfortably poking out of the fretboard. Sometimes they can be quite sharp! This is often a result of the neck drying and slightly shrinking while the fret wire does not. They fret ends can be simply filed off minimally, or fully dressed for maximum smoothness.
A fret dressing is recommended when frets have become worn out in (and may also be causing string buzz or other problems). Other times, due to the natural settling of the wood, the fret height becomes inconsistent across the entire fretboard, resulting again, in string buzz or other problems. A new guitar may likely need levelling for this reason. When dressing the frets, all frets are seated and levelled to uniform height across the neck. Once levelled, they are shaped and polished.
A refret is necessary when frets are so worn out that they need to be replaced. Another reason for a refret is the player may prefer a different size fret wire. When refretting a guitar, the frets are removed and the neck is checked for straightness. If necessary, the fingerboard is re-surfaced (planed) before the new frets are installed. Finally, the frets are levelled to a uniform height, then reshaped and polished.
A fretless conversion is when the frets are removed from a bass or guitar and replaced with a veneer (see photos below).