Often Existential themes in literature and film appear in the form of characters experiencing and "existential crisis" of some type.  These themes go way back, all the way back, to the earliest stories -- Gilgamesh when Enkidu dies; Achilles when he faces his own impending death; Hamlet when his father dies, his mother remarries, and he, himself, is denied the throne -- but they are even more prevalent in contemporary literature, and you'll quickly learn to see the influence in all kinds of pop culture, such as the darker versions of Batman, Daniel Craig's 007...anywhere, it would seem, where the protagonist is stripped of his or her most basic assumptions, faces the absurdity of existence, and asks "what's it all about?"

Existential Questions
The word "existential" combines the two words "existence" and "essence"  ("exist-ential"), and this etymology tells us what an "existential question" is:  a question concerning the essence of what it means to be alive, such as:

What is the meaning of life? = What is the meaning (essence) of existence?

Who am I (essentially)? What is my true nature/essence? What is my true identity?
What is my greater purpose? How should I live my life?
What is death? What happens when we die?

Is there a god, and, if so, what is her nature?

In a world of moral ambiguity, a world of "gray" areas instead of moral black and whites, good vs. evil etc., how do I make moral choices?  How do I choose between two competing bad choices?  Or how do I choose between two competing good choices?

At the point of absolute crisis, the protagonist realizes he or she has no one to turn to for answers...but him or herself.  We, alone, must choose.

"Existential Crisis"

Most of us don't walk around every day asking these questions -- we just go about our day, simply existing -- but for nearly everyone, there will be times in our lives when the surface meaning is stripped away, usually by some tragedy, separation from those we love or our daily routines, from our normal "identities" and/or by confronting "the destroyer": death.

For example, if you have defined yourself as an athlete, and therefore being an athlete is what gives your life meaning, you'd probably face an existential crisis if you were injured -- or you simply grew old -- and could no longer compete in your sport.  Or maybe you are deeply, deeply in love and have planned the rest of your life around someone...only to learn they don't love you at all. The same happens for those who define themselves -- find their deepest meaning in -- as a parent, and then the children all move out and don't need you anymore.  Or you are a soldier fighting a war for what you were led to believe was a people who would greet you as a liberator and hero, but you learn they actually hate and want to kill you.

Perhaps your religion taught you that your god rewards the righteous and punishes the wicked, and so you lived your life righteously, only to have every member of your family wiped out in a meaningless genocide or accident etc. or you yourself or even your newborn child are visited by a terminal disease.

So existential crises force us to ask ourselves what our true nature is, our true meaning, our true essence as a human being, when the surface meaning has been removed.  In other words: if being an athlete is what gives your life meaning, what does your life mean when you can no longer be an athlete