Honestly, I think it may be a "sheild ability" type of bar, like, I dunno, how much you can reflect a spell back at an enemy, or, say, being able to create a gust of wind by sheild bashing the air similar to the Deku leaf in WW (which would be pretty cool). However, it could easily be a durability meter, and that could add a lot of depth to gameplay. Like many of you pointed out, it would give us stuff to buy on a regular basis. There could be a weak, cheap wooden sheild, a medium strength and medium priced iron sheild, and a strong expensive steel sheild. Also, there could be an expansive sidequest (which I always love) in which you could get an invincible sheild at the end. I think that would be a GREAT addition to the game, personally.