Also called the Risk Ratio.
This is a measure of the association between a predictor (or independent) variable and a yes or no (binary) outcome (or dependent) variable.
For a yes or no predictor, the relative risk estimates the ratio of the chance of the outcome being present when the predictor is present to the chance of the outcome when the predictor is absent.
When a predictor has several categories, one is chosen as the reference category and then each of the remaining categories is compared to that one.
For a numeric predictor, the relative risk pertains to a one unit increase in the value of the predictor, and the assumption is that this is the same no matter whether the one unit increase is from 0 to 1, 10 to 11, or 987 to 988 (this is the linearity assumption).