This is a modifier in favor of the character in question. It is marked with a + sign, but is not necessarily actually something you add. You have to interpret the Bonus to be an action favorable to the character. For example, Pick Pocket gives you a Bonus to your Luck roll. However, some of the descriptions of abilities and skills (notably in the Class description sections of the various Course Catalogs) can be a little confusing. The + sign simply represents a Bonus as outlined here.