This is an ability word, introduced in Scars of Mirrodin. It is a word used to mark abilities which are all tied together due to the requirement that you must have three or more artifacts on the battlefield under your control; if you do - then this creature or artifact does something as a response.

Carapace Forger says:

Mox Opal says:

Metalcraft - : Add one mana of any color to your mana pool. Activate this ability only if you control three or more artifacts.