JPlay shuts down processes on your system to try to eliminate noise and timing issues from other processes running. It connects to another player, like J River or Foobar, for file management and UI. It seems to work best for people who have a lot of processes running. Some people hear a big difference, others do not. The J River people tried it on their systems and did not hear a difference. From what I can tell it is very system dependent as to whether it helps or not and, if so, how much. Its best performance mode completely shuts down most of the system, so you set up what you want to play and let it go. You essentially have to reboot to play anything else. Lots of people swear by it and lots think it does nothing.
Fidelize also shuts down processes but it runs as a separate process but does not directly interact with the player.