The line comes from Tron Legacy, the 2010 sequel to the original Tron, in which Jeff Bridges returns as Kevin Flynn, the programmer trapped inside a digital world. The brief yeah is spoken in a moment of enthusiasm, and because it is so short and easy to recognise, it has been lifted from the film and reused online as a punchy reaction sound. It still works years later because the delivery needs no context: one word is enough to signal agreement, excitement or approval.