This is the chance that a person who has been exposed to the condition you are interested in develops the outcome in question compared to a person who has not been exposed to the condition developing the outcome. This is typically used in prospective studies.

If the RR=1,  the risk of developing the condition is the same in both groups. If the RR>1 there is an increased risk for the exposed group and a RR<1 means there is a decreased risk for the exposed group.

Example. Smoking (exposure) vs. Lung Cancer (outcome)

RR= Rate of someone that smoked developing cancer/rate of someone not smoking developing cancer = (15/60)/(6/30)= 1.25

Interpretation: The risk of someone who smoked developing cancer is 1.25 (or 25% higher) than someone who did not smoke.