The mission is an overarching purpose of why you're doing something. That purpose drives your entire being and productivity towards that end goal.
The mission is highly individualistic. Some people want to make gazillions of dollars. Some want to save the world. There is no right answer, as long as that answer is yours.
Getting to your mission is hard work. You have to think about it over a long period of time to come to the realization of your mission.
Your mission usually has some ties to your past behavior. One way to surface your life's mission is to tie up your previous experiences and see if there's a trend there. Oftentimes there is. Oftentimes it's not obvious but it's there.
One way to test if the mission you discover is your true mission; it should hold true if you had no other constraints in life. In the absence of money, family, friends, and other needs, the mission should be the same. Another way to test a mission is whether you can wake up everyday and serve that same purpose.
Aligning a person's mission to their daily work is one of the most powerful forces of productivity and well-being. Searching for one's mission is an endeavor that is worth spending time on.