Physical shield is a spell which, when activated, protects you from any attacks (melee, magic, ranged, etc.) and each time you get hit, it reduces your mana. Hope it is a good enough explanation.
As for how to get it, you should be able to get it at the experience portal (green one) in the guild. Can't remember which specific category as it's been a while, but I remember that it is in the magic spell section.
As for activating it, if you map it to your function keys, just press that key and it will be activated. If you didn't assign it, you will have to press the 'Shift' key and scroll down until you see it and press the left/right mouse button to activate it. Note that this is for the PC version. I'm not sure how it works on the console version.
