Let’s assume for a moment that we compared number of hours studied by a group of college students to their final exam scores. We might assume that those who studied longer hours would score higher on the final exam. That is considered a positive correlation.

Now, let’s think about hours exercised per week by a group of college students, and how that might relate to percent body fat. More hours exercised would reasonably lead to a lower percent body fat, right? (Actually, that’s more of a thesis question, but for the sake of my example let’s say yes.) That is considered a negative correlation.

Further reading here.