Guard, is a Persistent Team Power-up that will be carried by the player until player is fragged. Team Power-ups are not limited by time and grants their abilities passively. It doesn't drop out from player's hands when he or she got fragged it sticks to the player under all circumstances once picked up.
In Team Arena it works pretty much like a no period limit Regeneration Power-up. Doubles the player's usual health and armor limit on pickup, up to 200 health and armor (considering player have no handicaps) and gives health regeneration which will regenerate slowly and won't let health drop below the new, doubled limit. Player may replenish lost armor points by picking them up later. In case Guard possessor have it, he or she can improve health even further (225 max) by using a Personal Medkit. Any health value above 200 will not be regenerated back. Guard Power-up, generally preferred by heavy assault type players to dive in the enemy base.
In Quake Live, Guard Power-up is modified to cut 50% of incoming damage, kinda negates the new Doubler in Quake Live. Guard is pretty weak when used alone, possessor should find armor pickups to play offensively or avoid close quarter combat and stay on defending the base.