It expands the Aincrad storyline, which many fans found to be too short and had a lot of wasted potential.
Creator Reki Kawahara himself wrote the light novels, and it features a better look at what happened over at Aincrad when the Sword Art Online incident happened.
As for the anime itself, they announced the Progressive light novels’ anime adaptation last September.
Yoshitsugu Matsuoka and Haruka Tomatsu are both returning as Kirito and Asuna respectively.