Personal Shields are devices built into harnesses on some EVA suits and all combat body armor. They function by absorbing or wide disperal reflection of incoming energy attacks or radilogical events. Personal Shields can be over loaded by straining them to thier maximum limit in one minute. Unlike a starship that regenerates every second these shields regenerate one point every minute. Most background radiation is deflected with no power loss. However the shield battery will not last forever and will maintain a shield no under duress for three hours. Each point that is restored takes three minutes off the battery. In a stroke of genious the UNSC did create a port that allows the batteries from energy weapons to be used in an emergancy. However they are not as powerful as a shield battery and they will exhaust in one hour.
Shields cannot stop bullets and HV attacks. They can absorb the concussive force of an explosion.
When your shot in the shields. The shield will absorb up to the ammountof damage its rated for then fail allowing the rest though. EG your have 25 points of shield left and the alien hits you with a plasma bolt for 30 points of damage. 25 is stopped but the other 5 gets through and hits you.