This mosaic has a complex historical context, with many laters of meaning and interpretation possible. I think its fair to say that it is not the most artistic or imaginative of the Byzantine mosaics, but it does convey one of the more interesting messages.
Showing the Emperor Justinian and his entourage, leading and joined by the pillars of the State, the Church (on the right), and the Army (on the left) attending personally to the high altar at Ravenna’s new major church, San Vitale’s.
Justinian represents the bridge between the ancient Roman Empire and what we now call the Byzantine Empire. He can be considered “the last of the Romans” but he can be called “the first of the Byzantines.” Given the cultural importance of his reign, I tend to hold more with later view. But of course, the Byzantines would have been insulted to not be considered the rightful continuation of the Roman Empire, and strove for centuries to reestablish the Roman Empire in its full, Mediterranean-wide glory. Ravenna, a capital in the late Roman Empire was at the period the principal political city in Italy during the late ancient and early medieval worlds.
Justinian was able to deploy the greatest general of the age, Belisarius, who just manages to reconquer Italy from the Ostrogoths. Frankly, reestablishment of the Empire was largely beyond the resources and organizational abilities of Justinian’s state, so the Byzantine rule in Italy was constantly under threat, and needed constant reinforcement. It became an “exarchate” a sort of military province based at Ravenna.
The mosaic then, represents an attempt at a imperial return. Its a statement of reestablishment. The actual faces in the portraits, probably correspond to actual people. The figure just to our left of Justinian is thought to be Belisarius, and the figure on our right of Justinian is Maximilian, Bishop of Ravenna, a key and energetic figure in the Exarchate.