Absorption (or Abs %) is the average damage reduction provided by a character's equipment. Absorption is represented as a single value visible when using the stats command, calculated by adding the average absorption of each armor piece.
Absorption works on a per-hit, per-location basis. Each piece of armor reduces damage by a hidden value not visible in game. The overall Abs % is the sum of the absorption of the individual armor pieces multiplied by the relative hit location of each area.
A burnished metal breastplate has an absorption of 81%. The body area receives 49% of hits in combat, on average. Therefore, a fully repaired burnished metal breastplate will add 39% to the overall absorption Abs % shown in stats (81% multiplied by 49% equals 39.69%. Partial percentages are not shown in stats.).
Armored locations hit in battle will suffer absorption damage to the equipment there, reducing the effective defenses over time. This damage can be repaired by going to an armorer and using the mend command (mend xxx, where xxx is the name of the item).