the shield power level detemines the shield regen and resistence. with 50 shield power you have the stats showing in the tooltip, every point over or below, will cause 4% regen add or loss. and the regen works too in combat.
The power level calculator includes a box that will calculate your power based shield damage resistance from the power level (which it also calculates based on your skills, equipment, and settings). And this resistance is applied to all damage (or at least to all energy damage, I'm not 100% sure on that). Also note that different shield damage resistances will stack multiplicatively (eg. for a 20% and a 10% you take 0.8*0.9=0.72 and so the total resistance is 28%).