What is "Grammelot"?

What is Grammelot? And why do animators need it? Sometimes you want your characters to speak, but what they are actually saying isn't really that important.  Much more significant is the tone, and what that tone tells us about the character doing the speaking.  Think of the adults in the old Charlie Brown feature films; they spoke in a dreary unintelligible monotone.  We didn't know what they were saying, but we didn't need to - their tone of voice told us everything we needed to know.  And the content of their speech was far less important than what the Peanuts' kids had to say.