Defense fields were a type of deflector shielding that was employed by late 23rd century Federation starships. They were raised during yellow alerts, and protected the bridge area as well as other critical parts of the ship. Defense fields were very weak and could be taken down by a few phaser shots from another ship. (Star Trek II: The Wrath of Khan)
This type of localized shielding only appeared in Star Trek II: The Wrath of Khan. In previous and subsequent films and episodes, full shields were typically raised during yellow alerts. The graphic depicting the fields had the title intruder alert, suggesting that the fields were used to protect sensitive areas of the ship from potential intruders.