Chatter can be thought of as a run of small, light sounds that keep coming — quick, close together, and none of them heavy. A bird in a tree makes that run. So do teeth in the cold, a shaking machine, and a person telling you about their day. The picture does not say the sounds are worthless; it says each one is small and there are a great many of them, so what you notice first is the run itself rather than any single sound in it.
From the word’s history
Wiktionary records chatter from Middle English chateren, earlier cheteren ('to twitter, chatter, jabber'), described as of imitative origin — the word was made from the sound itself. That fits the picture: the mouth makes a run of quick light sounds just saying it.
Wiktionary 記錄 chatter 來自中古英語 chateren、更早的 cheteren（意思是啾叫、喋喋不休），並註明它屬於擬聲來源，也就是照著聲音造出來的字。這條線索正好對上上面的畫面：你光是把 chat-ter 念出來，嘴巴就已經做了一串又快又輕的短音。
Wiktionary entry for 'chatter', etymology 1 (kaikki extract source_ref kaikki:3f6f69a190036ae4:878397190:38030), supplied in this evidence packet
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