Back in the days of TMP and early TNG, there was a distinction between "shields" and "deflectors." Shields were a forcefield that covered the hull of your ship and prevented radiation and destructive energy from touching it, while deflectors were a type of warp field that could propel massive objects -- asteroids, projectiles, etc -- away from your ship before they hit you. Over the years, the two concepts have been conflated as being the same thing, but the technical justification makes sense: if you can use your engines to propel yourself at FTL velocities, you should be able to propel your enemy's missiles and torpedoes away too.