The Vortex Shield is a special ability equippable for Titans that allows them to stop enemy fire such as rockets and bullets in their tracks, and is able to send the projectiles right back to the enemy. This effect is most likely orchestrated by a series of powerful magnets in the hand (as shown in the picture by the black and white device on the Titans forearm), but does not extend to the lightning bolts fired by the Arc Cannon.
This ability is best used when surprised by an enemy titan, or when an enemy titan has activated their own vortex shield. In this way, it is possible to catch projectiles already caught in the enemy's vortex shield and send it back before their shield can recharge.
Most effective against munitions and rocket ordinance, the Vortex shield can essentially catch projectiles for a return to sender. If caught off guard by a flanking opponent, a Pilot can quickly focus a Vortex shield to protect their Titan from incoming fire. The lock-one feature of the Archer Heavy Rocket means that a quick thinking Titan pilot can turn towards the incoming missile and catch it with a Vortex before it deals considerable damage. It is, however, less effective against certain Titan weapons such as the Arc Cannon or the Plasma Railgun, due to the "charging" feature of both weapons, meaning enemy Titans can wait for the Vortex shield to run out. The Triple Threat can also pose a problem, as any grenades that aren't fired directly at the shield can explode at a Titans feet, dealing damage. The Stryder possesses the most benefit of the Vortex shield, as its light armor is mitigated somewhat by the Vortex shields ability to catch most incoming munitions.
It is possible that the Vortex Shield may be a reference to the District 9 Exosuit or the Return To Sender Vigor from Bioshock Infinite. Both of these are capable of collecting incoming enemy fire and firing them back at enemies, though both collect the bullets into a sphere first.