According to the gospel of John in the Christian Bible, after Christ was crucified, a Roman centurion by the name of Longinus pierced the side of the body to prove it was dead, producing a rush of "blood and water." The spear, thus covered in the blood of Christ, would be a potent relic -- perhaps the most potent relic in medieval Christian belief after the True Cross.
The Holy Lance was "discovered" (or, perhaps, rediscovered) in 1098CE by a peasant, Peter Bartholomew, during the First Crusade siege of Antioch. He claimed to have been told by God in a vision that the Lance was under the floor of the cathedral in Antioch; when the paving stones were pulled up the fragments of a spear were found, and it was acclaimed as the spear mentioned in the Bible. Thus fortified with a powerful relic, the flagging morale of the Crusaders was restored and a great battle was fought against the besieging Turks, breaking the siege and establishing Antioch as a Crusader stronghold.