A Geth shield reacting to an attack.
Energy Shields, or Kinetic Barriers, are a common form of protection used throughout the League Territory. They can be applied to soldiers, ships or other things, and are useful as they are not as heavy, but also not as strong as, armour/plating.
A very powerful, but temporary, form of shield, the Bubble Shield.
Energy Shields come in many different forms but the basic principle remains the same. Small emitters within a suit of armour or plating detect objects moving at rapid velocities and project a barrier, deflecting the object and preventing harm to the user.
Dalek Shields are some of the most powerful in the Galaxy, but even they have their weaknesses
Shields provide effective protection against bullets or other ballistics but are vulnerable to rapid fire and can be shut down by electronic attacks. They can also be overloaded by natural sources, such as magnetic radiation from a powerful star. Since the advent of The League's latest shielding system, which ignores the properties of each individual projectile, many members have scrapped their traditional Plasma weapons in favour of rapid-firing or ballistics, or even Lasers, as Plasma tends to travel too slow, overheat weapons quickly, and has no additional effects on the new system like it did on the old one.
Spartan and Elite Shielding was particularly vulnerable to plasma.
The speed at which Plasma projectiles travel makes them a very easy item for shields to deflect, and so they deal minimal damage to the system.