Deflector shields are the energy barriers that surround a starship to absorb or deflect weapons fire, radiation and other hazards.
Starships raise shields when entering combat or hazardous environments. Shield strength is tracked as a percentage, and sustained fire drains the system until the barrier collapses. Ships must usually lower shields to use transporters, which gives attackers a tactical opening.
Shield frequencies can be modulated to resist particular weapons. Opponents that adapt quickly, especially the Borg, can learn a ship's shield frequency and render the protection useless, forcing crews to rotate frequencies constantly.