Something extracurricular takes place in addition to your regular school or work duties. Being involved in extracurricular activities in high school — like sports or journalism — can help you get accepted to the college of your choice.
Extracurricular is a combination of the Latin prefix extra- meaning "on the outside" and the word curriculum, meaning "a running, course, career." Something that is extracurricular is on the outside of your course or career. When you get involved in an extracurricular activity, you are going outside of what's expected. Extracurricular can also refer to a spouse's adulterous activities, taking place outside of their marriage commitment.