It is the Pledge Grove. If you have Cobalion, Terakkion, Virizion and Keldeo in your party, bring it to this place then Terakkion, Cobalion and Virizion will teach your Keldeo the event-special move Secret Sword, which allows Keldeo to transform to it's Resolute Form if the said move is learnt. Other than that, it's nothing, really.
Keep in mind that your Keldeo MUST be an event Keldeo in order to trigger the Secret Sword event. A hacked Keldeo won't work.
Hope I helped!
Source (Pledge Grove)