More than anything else, Excalibur symbolizes power. Whoever wields this mighty sword is universally recognized as the rightful king of England. But as well as earthly power, Excalibur also embodies a more supernatural force. It's not enough to wield power; like the sword itself, it must be used responsibly. Excalibur, then, symbolizes transcendent moral values against which the actions of the respective kings of England are to be judged, and more often than not, found wanting.
That's certainly what happens in the case of Arthur Pendragon. He loses sight of what his responsibilities as king entail, as can be seen when he turns a blind eye to the illicit affair between Lancelot and Guinevere. Arthur is such a firm believer in chivalry that he becomes blinded by it, refusing to contemplate that Lancelot would ever dream of violating the knightly honor code. This willful blindness constitutes a gross dereliction of duty, an abnegation of the ominous responsibility symbolized by Excalibur, and it eventually leads to Arthur's downfall and the fall of Camelot.