Self-Actualization in the words of the man who coined the term, Kurt Goldstein, "It is the tendency to actualize your own individual capabilities in the world". From this description, it can be understood as the state of mind in which you know about yourself and know what you can do and apply those things to better the society you are part of.
Abraham Maslow added on to this idea by adding Self Actualization into his Hierarchy of needs and in fact put it on top of the pyramid. He describes Self-Actualization as "a desire for self-fulfillment, to find your potential and to reach what you could potentially be". It is attaining the desire to become "one's true self".
And it is finding your true self and being able to realize your potential which you should be concerned about in this world of doubts and questions, in a world where you are unsure whether you can do something or not.