It's how much you get staggered ... I think.

Its a stagger bar. If it fills completely you get staggered and stumble around and cant hit things for a moment.

I'd add it's dependent on the size of your health pool. More health, harder to stagger.

it fills faster if you are taking consecutive hits quickly, once filled you stagger back a few steps. i believe damage takes part also, more notably critical strikes that land more damage than the shield can mitigate. higher the crit the more stagger bar is filled.

edit: this is most noticeable in the plains when up against Fulings

It's a heart? I thought it was a kite shield.