Adonizedek was king in Jerusalem at the time when the Israelites invaded Palestine. He formed a confederacy with the other Canaanite kings against the Israelites, but was utterly routed by Joshua when he was engaged in besieging the Gibeonites.
The name Adoni-zedek follows a pattern of the great Melchizedek from the days of Abraham. Literally the name means "Lord of Righteousness (or Justice)". This is much like Melchizedek, which means "King of Righteousness." The similarity may show a connection to the early history of the city. If anything, it demonstrates a decline in the authority of the leader at Jerusalem. What had been a king's reign had become that of a "lord."
Extra-canonical verification of Biblical account
Among the Tell Amarna tablets are some very interesting letters from Adoni-zedec to the King of Egypt. These illustrate in a very remarkable manner the history recorded  and indeed throw light on the wars of conquest generally, so that they may be read as a kind of commentary on the book of Joshua. Here the conquering career of the Abiri (i.e., Hebrews) is graphically described: "Behold, I say that the land of the king my lord is ruined", "The wars are mighty against me", "The Hebrew chiefs plunder all the king's lands", "Behold, I the chief of the Amorites am breaking to pieces." Then he implores the king of Egypt to send soldiers to help him, directing that the army should come by sea to Ascalon or Gaza, and thence march to Wru-sa-lim (Jerusalem) by the valley of Elah.