For example, this answer makes the following statement:
What is true is that [vaccine effectiveness] in people aged 65 or older is 9%, with a 95% confidence interval that spans from -84% to 55%. They suggest interpreting these results with caution.
I want to ask a question like "What does it mean for a confidence interval to include negative values?" because I don't understand the statistics needed to make sense of answers like that. But that's not about a specific claim, so I don't know if it's on topic here. Can I ask a question that basically means "can someone explain an idea to me"?