The Seven Lights, or "Seven Guardians of Light" is small group of Keyblade wielders who are destined to oppose the 13 darkness and create the X-blade in the process. Only six of the seven identities have been confirmed: Mickey, Sora, Riku, Ventus, Aqua, and Terra, as listed by Mickey himself in Dream Drop Distance.
During Dream Drop Distance, Master Xehanort revealed that he knew his attempts to create X-Blade through the reunion of Ventus and Vanitas was a hasty attempt and that he had always been searching for a true way to forge it. For this reason he enticed Maleficent into capturing the Princesses of Heart, with his Heartless Ansem overseeing her progress, while his Nobody Xemnas created Organization XIII so his splintered selves can obtain the Lights and Darknesses, respectively. But, as Ansem failed to possess Riku and Roxas returned to Sora, Xehanort's new organization would be one member short so he decided to use Sora. This attempt, too, failed thanks to Lea, Donald and Goofy.
As the 13 Darknesses return back to their timelines while short one member, Xehanort tells his opponents that the battle between his Darknesses against their Lights will happen and they they will meet again at the fated place. Because of this threat, fearing Xehanort would go after the Princesses of Heart if they did not go through with it, Yen Sid and Mickey resolve to gather the Seven Lights. While Terra was previously serving as Xehanort's vessel and would be suited for the role of one of the Darknesses, Xehanort hints that Terra is no longer in his possession when he tells Mickey that, once he claims Sora, he will possess two of the Lights. These "two" are most likely referring to Sora as well as Ventus, whose heart still resides inside Sora's. However, the secret ending of Dream Drop Distance reveals that Yen Sid has Kairi brought before him to teach her to wield her Keyblade, likely making her the seventh Light.