This is a term associated with rock music, notably punk rock as this is what a given band member would yell out right before starting a song at a concert. It was made famous by the New York punk rock band The Ramones, whose bassist Dee Dee Ramone would always yell out before every song
and sometimes even during them.
"Dude, you think Dee Dee's voice gets tired from yelling 1-2-3-4! all the time?"