An unlocked subdomain is one that a character can now choose and isn't added to the normal number of domains a character possesses
The racial trait Kobold Faith says
You gain a +2 trait bonus on Diplomacy checks when dealing with other kobolds. Additionally, if your deity grants the Artifice, Earth, or Trickery domain, then your kobold faith unlocks a corresponding subdomain: Trap, Radiation, or Ambush.
But the term unlock isn't a technical term. However, in its introduction to the Kobold Faith trait, Kobolds of Golarion (2013) explains
Even kobolds who worship the same deities as surface dwellers put a kobold spin on their religious practices. This results in some rare subdomains which are accessible only to kobold followers of that faith. The following race trait grants access to the subdomains detailed below.
So here unlock means able to be picked. That's because Domains on Subdomains says that
A cleric who chooses a subdomain must have access to both the domain and its subdomain from her deity. If a cleric selects a subdomain, she cannot select its Associated Domain as her other domain choice (in effect, the subdomain replaces its Associated Domain). Subdomains are treated as equivalent to their Associated Domain for any effect or prerequisite based on domains. If a subdomain has two Associated Domains, the cleric can only select the subdomain for one of her domains.
Emphasis mine. As all three of the Trap, Radiation, and Ambush subdomains were introduced in Kobolds of Golarion, there were only a few ways to allow existing gods to grant those subdomains, and designers appear to have chosen the trait Kobold Faith rather than, for example, issuing errata or updates for all deities that should've been granting those subdomains all along, in essense saying, "You want a kobold with the radiation subdomain that badly? Take a trait."
The short version
Taking the trait Kobold Faith allows a kobold that is already allowed to pick the domain Artifice, Earth, or Trickery to pick instead the subdomain Trap, Radiation, or Ambush, respectively.