Pick your choice of hoot'n and holler'n :>) Basically any degree of emotional yelling similar to what you'd hear at a sports event today.
You must put it into perspective of the rigid discipline of Napoleonic Infantry tactics where soldiers were trained to stand shoulder to shoulder and follw by rote, multiple step commands to load aim and fire their weapons stoicaly without much noise other than that of the person yelling the commands.
When well trained and disciplined Union soldiers heard the yelling it was a suprise and often seemed to them that they were facing a ragtag uncontroled mob. This was "dishonorable warfare" in the classic sense of the day. Of course, throughout the course of the war battlefield tactics changed tremendously and we saw the evolution of trench warfare. This was then polished to a whole new state of use during WWI in Europe. Nonote that by then troops woould come out of the trench with a yell and a whoop to charge across no man's land. So in a way the Rebel Yell was just a precursor to WWI, or the expression of the hightened sense of emotion that running pell mell into the face of death gives you.