Easily one of the most famous stories of Jewish scripture is that of the fight between David an Goliath. Its imagery resonates with people throughout time; a small, gentle boy triumphing over a very big, fearsome foe. In chapters 16 and 17 of First Samuel we see the first appearance of David and his quick, surprising battle with Goliath the Philistine champion. Taken in the context of the book and the time in which it was written, we'll find that this well-known story doesn't mean exactly what it's interpreted to mean in popular analysis.
For a long time now, David's victory over Goliath has been the subject of a lot of artistic license, much of which has resulted in embellishment and misinterpretation. First and foremost among these inconsistencies is the idea that Goliath is a giant. Granted, the text makes it a point to mention Goliath's height (a little over six cubits) and for the time that would have been significantly taller than most people, but the text also never refers to Goliath as being a "giant" in any sense. A cubit is roughly estimated as the length between one's hand and one's elbow. While far from exact, a cubit would average around the modern concept of 12 to 14 inches. The stature of Goliath has long been exaggerated, as has the apparent age of David at the time of the fight. Goliath was certainly a big guy and David was rather young, but the fight in the text is between a six foot tall Philistine and a boy in his mid-to-late teens, not between a mythical giant and a child.
As for the symbolism of David defeating Goliath, it is often interpreted as the greatest underdog story of all time. This, again, is more of an attempt to apply a modern sensibility to an ancient story. David was by no means a true underdog and this isn't a story about an unlikely hero overcoming seemingly insurmountable odds. David was publicly anointed by Samuel in a scene during which God explains that appearances such as height and age don't really matter. As soon as Samuel pours the oil on David, David feels the certainty of God in him. So, when David volunteers himself to battle the strongest warrior of the Philistines he doesn't do so as an underdog, he does so as an individual protected by God.
The lesson here is essentially to not make assumptions about the people around you. As God says to Samuel, "Man looks at the outward appearance but The Lord looks into the heart." One of our society's greatest thinkers, Martin Luther King, said so much when he talked about judging people based upon the content of their character. This book, the preeminent ancient guide to political leadership, attempts to impress upon its readers the importance of thinking beyond immediate, fleshy ideas. Youth, pretty eyes and ruddy skin are just descriptions of David's body. In his heart, where it counts, he already has the character of a king.