I don't remember the shield power-up from the original temple run. I try to pick it up like all power ups, but it tends to fade away without seeming to have done anything. I assume it shields you from some traps, but I'd like to know the exact list.
I just played a few rounds to confirm:
The shield prevents death from all solid objects, including the rotating spike cylinder, and fire. It also prevents you from tripping on the tile piles that occassionally block either side of the path, and from tripping during mistimed jumps where you fall short and hit the incoming ledge. All of these objects, once hit, deactivate the shield.
When in the mining tunnel, the shield DOES NOT prevent you from tilting your mine cart into the tunnels blocked with boulders (the boost does).
The shield does not save you from falls, water hazards, or broken mine track.
The first Temple Run also included a power-up with the same properties but was called Invisibilty.