Conversi (usually translated as ‘lay brothers’ in English, though that term has fallen out of favor) were members of monastic communities who had taken vows but not received the sacrament of Holy Orders. They were monks who entered monasteries later in life, typically to perform some kind of penance, and were often unlettered peasants. They were responsible for menial labor and sometimes managed secular business for their communities. At Fossanova (and elsewhere) they lived separately from the other monks, and thus had their own entrance to the church.
The capitals here show an interesting simplification of the Corinthian order, with a single stylized acanthus leaf appearing at the center of each facet.