A deflector dish is a component of many starships used to project a shield around a ship to deflect small objects and space debris which could damage a ship. It is a particularly crucial component of a starship as even the smallest particle could cause serious damage to a ship travelling at high velocities, particular at warp speeds.
On Federation starships the deflector is usually located at the front of the secondary hull and typically glows with a blue energy. Because of the deflectors ability to produce large amounts of energy and particles to very tight specifications they have often been utilised with minor modifications to help solve a variety of tasks; the Enterprise-D's deflector was once modified to channel an extremely large amount of energy in the hopes of using it as a weapon against the Borg (TNG episode: The Best of Both Worlds). The USS Defiant's deflector was once modified to be used as a single shot phaser and the USS Voyager’s deflector has been used to open a rift into fluidic space (DS9 episode: Starship Down and Voyager episode: Scorpion).
External Link
Navigational deflector article at Memory Alpha, the wiki for canon Star Trek.