The arched frame, door or window opening, portal, most often profiled, was decorated with ribbon friezes (Romanesque) or figures (Gothic).
View this one-story house plan with an archivolt above the entrance arched door.
Archivolts are simple, without any decorations. But the richer the building, the more likely you will find an arch decorated with archivolt. In Gothic buildings, archivolts were decorated with figures of people or floral ornaments.