To depart is to go away or to choose a different direction. If you are catching a plane, you are soon to depart. And, if you are wearing a kilt instead of jeans, you depart from your usual look.
Airports and train stations have gates for arrivals and departures. Airplanes and people coming in "arrive" while those going out depart. You also can depart without going anywhere, as when you change your mind and depart from an earlier decision. To "depart from the norm" is an expression for doing something different than expected, like giggling during a tear-jerker of a movie.