In Infinity, Damage is the capacity of a weapon, Attack, Special Skill, piece of Equipment, Hacking Program, etc. to harm or impair its target in any way. The Damage value, represented by a number, is indicated on the weapon's profile or in the rules for the specific Special Skill, Equipment, etc.
However, this Damage is not usually applied to the target as is. Most often, the target has a chance to avoid harm by passing an ARM/BTS Roll to ignore the Damage.