This root appears in many English words related to voice and speaking, such as:
* Vocal: relating to the voice
* Vocabulary: the words of a language
* Vocation: a calling or profession
* Vocational: relating to a particular occupation
* Evocative: bringing strong images, memories, or feelings to mind
So, "vociferous" literally means "full of voice," and refers to something that is loud and noisy, often in a forceful or insistent way.