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. The Deflector dish was also used as lightning rod to avoid the USS Voyager from being separated by temperal time barriers.(episode: Shattered)