A fret end file is used to file the ends of the frets that stick out passed the edges of the fretboard. Fret ends that stick out too far will feel sharp and can even cut your hand as you play up and down the fretboard. A fret end file consists of a machinists' file that is glued or attached squarely to the bottom of a block of wood or plastic. The file is the placed on the edges of the frets and run up and down the edges of the fretboard until the frets are filed smoothly.
There are two main causes for sharp or long fret ends.
Sometimes cheaper guitars' frets are not installed properly. Usually when frets are installed, they are oversized--i.e. the frets hang off the fretboard on each side slightly. Then the frets are trimmed down to the edges of the fretboard with a pair of fret end nippers or wire cutters. Once the frets are close to the edges, they can be filed flush with the edges of the fretboard. If the frets are not filed flush, they will be sharp.
The most common reason for having sharp fret ends is weather conditions. If your guitar plays fine one day and the frets feel sharp and awkward when you play it a month later, it usually means the weather has had something to do with it. Humidity or lack there of, will cause your guitar neck and fretboard to swell and contract based on the amount of moisture your neck is absorbing from the air. Your frets, on the other hand, are made of metal and are not as susceptible to the changing weather conditions. During a change from humid to dry air, your guitar neck will contract or shrink while your frets will stay the same. Thus, your guitar frets will stick out off the ends of the fretboard.
Tutorial coming soon!
Filing the ends of your frets is easy. Simply, take your fret end file and place it squarely on the side of your fretboard. Make sure that your fret file is on the fret ends and not the edge of your neck. You don't want to file down the finish on your guitar neck. Now file in one direction either up or down the side of the fretboard until you can no longer feel the ends of the frets sticking out of the fretboard. You just want to file as much as needed. There is no need to file into the edge of your fretboard. That is it! Now your axe will be ready to play nice and smooth. For more information about filing fret ends, please see the fret installation page.