That does the shield mean that shows up by health? Sorry I can not explain better.

It means your weapon is'nt overcoming the enemies DT, get something with more DAM, a broken sheild means your gun is bypassing the DT by having more DAM the their armor has DT, this works both ways, so a blue broken sheild means your armor can't handle the enemies attack while a red broken sheild means the enemies armor is'nt handling your attack.

It means that your Damage Threshold is greater than the damage the enemies weapon is doing to you. It is a good thing, if it shows up next to your enemies health then you need to get a higher dmg weapon or armor piercing bullets for them.

Thank you.