A role modifier is an additional ability or property that alters a role's normal operation or adds a new element altogether. Generally, only one modifier is given to a role (if at all), but some theme games have given a role two or more modifiers in effective and creative ways. There are many reasons why mods use modifiers; balancing strong roles, improving/altering the dynamic of role combinations, creativity and other meta reasons. Strictly speaking, any type of trait can be attached to an existing role.
For a list of role modifiers, see this category.