King David playing the harp is a common representation in medieval art. This iconography is often used to symbolize joy and redemption, as well as to express prayer and love to God. It also reflects David's role as the composer/performer of the Psalms.
In art, King David's kingship is expressed not by a crown but by signifiers such as an imperial arch or his sitting on a throne. He is usually depicted playing a lyre or harp, called a kinnor in Hebrew. In some images, he is accompanied by percussionists, horn players and dancers.
In The Bible, it is said that the music of David's harp helped King Saul to be eased of the torment that an evil spirit had been doing to him and be comforted. This symbolism has been carried through history, with the harp of David becoming an essential part of Temple ceremonies.