's ability grants to the equipped creature an activated ability that allows the controller of the creature to tap the creature to deal two points of damage to a targeted creature or player. Because the creature is the source of this ability, a creature with protection from artifacts can receive damage from this ability (as long as the equipped creature is not also an artifact.) Compare this to . Blazing Torch's ability specifically states that the Torch is the source of the damage.
In most cases the source of an ability is the card itself that has the text that causes the game or players to take some action. Some examples of straight-forard situations like that would be the activated ability of , the triggered ability of or the static ability of . In all of these examples the source of the abilities are either the card or the permanent represented by the card. However there are some instances where the source of an ability can be in question, and where it can matter. For