Open string voicings are one of the first skills you learn in guitar. Essentially, open string voicings are the fundamental notes of unstopped, full strings. So, for instance, an open string is playing the G note and letting it resonate.
Learn more about open string voices and how to play them by watching this tutorial, which utilizes the sheet music for the song "Hall Goes West." Your instructor will play the open string voicings for you so you can get a sense of how they sound.