Primarily means "for the most part." If in the summer, you primarily work at a restaurant, this is the thing you do most, though you also might hang out with friends, exercise, and read books.
You can use the adverb primarily any time you'd use the word "mainly" — today, many people rely primarily on the Internet to get their news. This doesn't mean they never read newspapers or watch television news, but most of the time, they learn about current events online. The Latin root of primarily is primus, which means first — most important, most likely, or what you choose before anything else.