An HDMI cable alongside an optical audio cable: the optical audio carries audio to an external audio system (sound system receiver, headphones, computer speakers, etc.) separately while the HDMI cable carries video directly to the HDTV.
情報源: Rocksmith® Guitar Game – Setup & Support | Ubisoft® (US)
でそのOPTICAL PORTと検索してみると、なるほどHDMIの横にあるこのあまり見かけないタイプのポートに差し込めばいいらしい。