To vociferate is to shout or yell. People vociferate when they want to be heard.
Vociferating is a fancy word for shouting. A police officer vociferates by yelling "Stop!" at a suspect. In sports, coaches' vociferating is aimed at their players and the referees. Parents often vociferate when their children misbehave. If someone is talking very loudly, they're vociferating. Never vociferate in a library: a librarian will tell you to keep it down.