Ablative (Noun) : Able to disintegrate or be worn away at a very high temperature: eg. a thick layer of ablative material
I looked up the word "ablation", apparently it has things to do with fire protection. For example: "In spacecraft design, ablation is used to both cool and protect mechanical parts and/or payloads that would otherwise be damaged by extremely high temperatures."
So what it means, I bet, is that the 10-micron-thick silver coating protects the armor from fire and lasers, but with time and exposure to fire it wears out, it's ablative. The ablative surface is there to protect from fire by having the fire eat it up instead of the armor. So, Power Armor is fire resistant... until the 10-micron-thick silver coating is used up.