In Metroid Prime 3: Corruption, there are two main uses; control panels and hidden switches/locks. The first is simple enough, just bring up the visor and copy out the code by pressing the corresponding characters. For the latter, you'll need a "high-frequency beam", by which they mean the Nova Beam. In terms of enemies, it can not only reveal invisible ones, but on certain ones it can show weak spots which when shot by the Nove Beam, make for an instant kill.
To change visors, press and hold the '-' button on the Wii Remote to bring up the Visor Select screen. The X-Ray Visor mode is in the lower left area.
 How to Obtain
 Metroid Prime
 Metroid Prime 3: Corruption
Can be found in Command Vault at Command Center on the Pirate Homeworld. To get it, you'll need to destroy the Multi-Lock Blast Shield surrounding it by use of Seeker Missiles, after which it will drop down for you to acquire.