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It's the probability distribution. Kind of like a confidence interval. The thicker parts of the graph are more likely to be the true uplift. In your example, there's a tiny chance that the uplift is actually negative (the red area), but it's most likely closer to a 10-20% uplift

Thanks Jérémy, I'll stick to that forulation to respond to stakeholders and try to find a good stats book to better understand the underlying calculation ^^