The Key of David is a Messianic term referring to Jesus being a descendant of David in the flesh (Matthew 1; Luke 3). and His Position as The King on The Throne of David in the Messianic Kingdom.
Mark 12:37
37 David therefore himself calleth him Lord; and whence is he then his son? And the common people heard him gladly. (KJV)
Here, using the Psalm 110 with a rhetorical question, Jesus says that if David is calling the coming Messiah “Lord” as in Psalm 110, then how can He merely be a physical descendent of David? David knew that this would be a God-man who must be the one who would restore the Davidic Kingdom on earth (II Samuel 7:16; Amos 9:11-12; Matthew 19:28; Luke 1:31-33). Jesus directed this exposition of the prophecy of the Messiah from the Old Testament to affirm His Messiahship to the Jewish religious authorities (which they were rejecting) but “the common people heard him gladly”. Some religious rulers, such as Joseph of Arimathea and Nicodemus, no doubt did accept Him but Scripture only affirms that there were few.
https://www.sermonaudio.com/sermoninfo.asp?SID=102223165523224&fbclid=IwAR2o5ahC8dQf03WcQO6aYXEojD9EIyWS-seKVJ6rQlcKpYBEsTf8_IGtIIA
Content retrieved from: https://www.sermonaudio.com/sermoninfo.asp?SID=102223165523224&fbclid=IwAR2o5ahC8dQf03WcQO6aYXEojD9EIyWS-seKVJ6rQlcKpYBEsTf8_IGtIIA.