The song "Only One" is for the '2014 Incheon Asia Games', which the JYJ boys are the ambassadors of. It's a happy, energetic song that cheers on the athletes, urging them to look forward to the shining future.
The "Only One" MV also features various parts of Incheon as well, and it was created through the concept that all Asians would sing and dance to "Only One". JYJ said, "We hope that everyone can think of the Incheon Asia Games through the song, and that it can be an enjoyable song to follow."
Thanks to Lidia for the tip!