Refers to the variables used in a model to predict a response. This tag can also be used for $X$ variables in explanatory & descriptive modeling, not just predictive modeling. This same construct goes by many names in different contexts, including: independent variable, explanatory variable, regressor variable, covariate, etc. This tag can be used for any of these synonymous terms.
Questions with this tag should be about handling of aspects of regression (or analysis of covariance, etc.) that have specifically to do with the predictors.
Predictors are independent variables / covariates that are used in the calculation for a regression or an analysis of variance because they may explain some of the variance of the dependent / outcome variable or variables. There is a subtle distinction sometimes made for the specific term 'covariate': their effects on the outcome are not being investigated, but including them can reduce the error variance and thus lead to greater power / precision for the variables of primary interst in the model.