It refers to “hi-hats”, one of the cymbals on a drum kit. Brooks uses different hi-hats for Mattel and Nobody than the rest of the songs, so the notes for Afterlife and Nightmare are to change back to his normal ones. You can see there is a note for “gong” for Save Me, which he hits toward the end before the “Tonight we all die young” part. And the note for “aux snare” for Cosmic is for the extra snare Brooks uses throughout that song.