What does the theme "life as a journey" mean?

The theme "Life as a journey" is a very common one in literature, and if you stop to think about it, there is good reason for that. We all perceive our own lives as a kind of story, as we look back or forward and try to make a narrative of all that happens to us. Literature simply capitalizes on the tendency to see life as a journey. In literature, we let the journey act as the metaphor for a literary character's life. Let's look at some examples. 
The most famous American example of the use of this theme is probably Twain's The Adventures of Huckleberry Finn.  Huck and Jim's journey on the Mississippi is the journey of their lives, both striving for freedom, albeit in different ways: one for freedom from civilization and the other from slavery.
A more contemporary example is The Secret Life of Bees by Sue Monk Kidd, in which Lily and Rosaleen leave home, overcome obstacles, gain wisdom, and come to relative peace in a new place. As the plot takes them on a literal journey, their lives are a metaphorical one, too. 
Another contemporary example is Khaled Hosseini's The Kite Runner. Amir, the protagonist, must go on a literal and figurative journey back to his roots in Afghanistan to right the wrongs he caused and redeem himself for his various sins. "Life as a journey" is just one theme in this novel, but it is an important one. 
"The Road Not Taken" by Robert Frost is a good example of a poem with this theme. The narrator is traveling, comes to a path, and must choose between two paths in the woods.  He chooses the path "less-traveled by" (line 20). The road is a metaphor for the narrator's life, showing the reader that as one goes through life, one must choose one path or another, with one path frequently being more or less typical or popular than another. 
The movie Crash uses the journey as a metaphor for life, as several intersecting stories about racism and other "isms" are borne out through people literally and figuratively colliding while traveling on their respective paths. The story takes place in Los Angeles, a city of highways.
There is also music that uses the theme of life as a journey.  Frank Sinatra's famous song "My Way" does this. He sings, "I traveled each and every highway" (line 6) in the very first stanza. There are probably hundreds of songs that use this theme. 
It is helpful to think of life as a journey, as it can impose some order on what might otherwise feel random and chaotic. Literature reflects our need to do this, to put a character on the road and make something happen as a result. Other art forms do so, too.