Its basically a fulcrum that allows the clutch cable to be released when you release the clutch pedal. It makes the clutch cable make a U turn back towards the firewall after it enters the firewall.
The stock unit is plastic and two pieces. Any aftermarket peice is aluminum and one piece. It will allow for more consistent shifts since the stock plastic piece will bend supposedly.
The stock unit is also adjustable (raising the clutch pedal). An aftermarket unit must be used in conjunction with a firewall adjuster to work properly and allow that adjustment.