Shields refer to various types of defensive technologies that shroud the wearer in a protective field that renders nearly all conventional forms of damage nullified.  Shields are based heavily on advanced understanding of magnetics that modern science has not reached yet.  Using electro-magnetism to suspend a sheath of variously charged particles and ions around a person it provides a layer of protection unmatched.

They only trigger at certain speeds of impact or with various energy sources.  Thus a person can walk around normally and there would be no clue they are shielded until a bright flash of light enveloped the person after they were shot.

Change in combat

Shields heralded a radical new direction in combat.  Given that they are not easily destroyed unless through heavy bombardment, normal ground combat no longer was based on range.  In fact the only way shields are easily over-come is through melee weapons that are Ionic Inducing.  These weapons create specialized fields over the weapon that allow them to pierce through shields.

Shields are largely expensive and as a result not everyone has them.  Thus a fight might still open up with long-range combat, and if the tell-tale sign of a shield – brilliant flashes of light – are seen then opponents close into melee distance.  As a result, a ‘well equipped’ fight would likely have a conventional long-range weapon, a shield, and an I.I. melee weapon.  They would be protected, be able to fight at a distance, and close into the only practical way individuals with shields can be stopped.

Use in space battles?

Ships naturally have shields in space, the largest difference is there is no ‘closing to melee’ range in space.  As a result battles are much more on the ‘outright bombardment’ option.  Remember, shields CAN be destroyed, and the damage they take is mostly impressive on the individual level.  When ships are capable of blowing away mountains, ionic fields are not as strong.  This is not to say they are meaningless, quite the contrary.  However in space fights they are must less of an obstacle so much as an annoyance or a nice insurance depending on point of view.