Shields in the GearHead universe are arm-mounted protective barriers used to block attacks. All-round force field technology is underdeveloped, even in L5. However, the phase-effect, used in the Phase Cannons of Radcliff and Chimentero fame, is somewhat analogous to projected force technology, so future researchers might figure it out some day.
Several varieties of shield exist, both for mechs and for people. Most shield use probably takes place at the SF:2 (mech) scale, where beam shields offer a lightweight and effective countermeasure to most forms of physical attack. The GH2 Ice Wind and Daum are fairly early-game mechs that use beam shields. Several mechs, especially in GearHead 2, use physical shields instead. Though physical shields are often significantly massier than the 0.5t of a beam shield, a significant amount of that mass is generally a weapon of some sort. The Atgeir and several Savin models equip physical shields.
Shields are also available at the personal scale. The Riot Shield is one example; the Ceram Shield or Buckler may also suit your needs.
Though you can equip weapons and shields to the same arm, the weapons on/in a shield-bearing arm will be less accurate. Several SF:2 physical shields have weapons installed in the shield itself; attacks using those weapons do not suffer the accuracy penalty. Beam shields will do severe damage to any physical weapon they block--including mech limbs. Consider installing & consulting Target Analyzer software or otherwise attaining familiarity with your targets before engaging in Mecha Fu.