Tertiary is another way of saying "third in importance," like socializing with co-workers being a tertiary reason for getting an after-school job — less important than, first, earning money and second, gaining skills.
To correctly pronounce tertiary, say "TER-she-err-ee." If you are the third child born in your family, don't be tempted to call yourself the "tertiary child." This means you are less important that your two older siblings. However, in some cases, tertiary does not have to do with ranking third — the Tertiary period marks the beginning of life for mammals, and in the United Kingdom, tertiary education means "college-level."