Tone is the author's wording of the article that conveys their feelings towards the subject. The lower the tone/sentiment score, the more negative the article.

These scores are averaged for each spot on the map, showing the associated color.

this is cool @prohobo! may I ask how you classify tone? I have a completely unrelated project (enterprise app with thousands of documents) on which I'd like to apply to. TIA.

I can't say specifically which algorithms are run on it, but GDELT runs sentiment analysis (https://en.wikipedia.org/wiki/Sentiment_analysis) on each article, and they have a new data set called GKG which is all about emotion mining.