King Arthur is raised by a relatively unknown knight, Sir Ector, rather than in the splendor of a royal court, as his lineage might predict. It is only by pulling the enchanted sword from the stone—a task at which all the best knights of the realm fail—that he proves both his blood and his worthiness to be king.
In many ways, Excalibur stands for the highest status of glory and honor that a knight can hope for, since the king of England himself has achieved it. But the sword also stands for divine fate, which human beings cannot hope to change. Arthur was always destined to pull out the stone and fulfill the prophecies set centuries before. In this way, Excalibur actually undercuts human pride and desire for glory, in its suggestion that humans must ultimately submit to what is already written for them. Arthur did not win Excalibur through any real merit or action of his own, but only because he was always destined to do so.
It should also be noted that there are two origin stories for Excalibur: the first is that this is the sword Arthur pulls from the stone, and the second is that Excalibur is given to Arthur by the Lady of the Lake some time after Arthur is already king. Malory includes both of these tales in his work and makes no attempt to reconcile them, which could lead to some confusion regarding how Arthur gained one Excalibur and then seemingly gains another with no explanation.