See: Correlation. Multiply by negative one.
You know those correlations that always rain on your parade, or that always assume the worst is going to happen? That’s not this kind of negative correlation, but those guys are real downers, aren’t they?
A negative correlation is a linear-ish relationship between two sets of data characterized by the pattern that, as the values in one data set increase by a certain value, the values in the other data set decrease by a certain (and usually different) value. Like, there’s a negative correlation between how long we’re on Reddit and the number of chores we get done.
Something highly negatively correlated means that one action has the opposite reaction of the initial one. Pounds of chocolate eaten this month is negatively correlated to the number of packs on an otherwise 8-pack stomach.