Goliathis a giant in the old biblical story David and Goliath and was depicted as the champion of the Philistines, traditional enemies of the Jewish people at that period of time - his size varies depending on the translations from anywhere around 6ft. 7 inches to over 9ft, though modern depictions often have him at much more monstrous sizes.
In the story Isreal and the Philistines were at war and Goliath would emerge daily to challenge the Jews to send their bravest warrior to defeat him and end the battle - however they were too frightened of Goliath's size and strength to do so and for over forty days Goliath pretty much kept the battle at a stalemate.
This would change when the young shepherd boy, David, arrived with food for his older brothers - he accepts Goliath's challenge and slays the giant via tossing a rock into his forehead using his trusty sling then proceeds to behead the fallen giant with his own sword - causing the Philistines to flee.
The tale of David and Goliath is often used to highlight how a man of faith can overcome impossible odds - much as David did when he slayed Goliath armed with nothing but a sling and faith in God.
The story of David and Goliath may also represent Christianity overcoming the Pagan religions - with the young David representing Christianity and the brutal Goliath representing the Pagan world, the stone David used to slay the giant may be considered a symbolism of the power of God in vanquishing all evils from the world.