If I remember right, it's a 'property' that pops up on really old shields, those from way back when Durable, Substantial, Massive, Fortified, or Indestructible were the categories - with the introduction of the visible stats and data, those got an extra 'label' at the top of their information, saying 'Durability ....%'
As far as I know, it makes no difference at all to their actual performance.