I had 3 different student age Bison fans yell "Roll Herd!" to me at the state fair and I almost didnt know what to say! My wife turned to me and said "is roll herd a thing? and I said "not that im aware of!"

I also saw someone wearing a "Roll herd" t shirt at the game. AM I MISSING SOMETHING?

Is "Roll Herd" a thing? and if so, how have I not heard about this?