This mosaic is one of four on the upper level of the basilica of San Marco (behind the loggia where the horses reside) flanking the main door to the plaza. From left to right facing the basilica, they depict the Deposition (entombment), the Harrowing, the Resurrection, and the Ascension of Christ.
The harrowing of Hell, also called the Descent into Limbo and the Anastasis, among other variants, took place between the crucifixion and the resurrection. Then Christ is said to have descended into the realm of the dead to reclaim the righteous and carry them to heaven.
By the early seventeenth century the original mosaics from the thirteenth century had badly deteriorated—the only remaining original mosaic is the one above the left portal, depicting the arrival of St. Mark’s body—and replacements were commissioned and executed in 1617-1618. The mosaic work was done by Luigi Gaetano based on cartoons by Maffeo Verona. The mosaics are made up of tesserae—small pieces of colored glass, stone, and enamel set in plaster.
The inscription appears to read “Ovis Fractis Portis Spoliat Me Campio Fortis” but I think the first letter is probably a Q and the V represents a U, as was formerly common. Then the Latin could be translated as something like “He who breaks down doors and carries me off is the mighty one.”