Wikipedia
Llan and its variants are a common placename element in Brythonic languages. In modern orthography, it is treated as a prefix, but was formerly sometimes written as a separate word. The (often mutated) name of the relevant saint or location follows the element: for example " Llanfair" is the parish or settlement around the church of (Welsh for " Mary").
The various forms of the word are cognate with English land and lawn and presumably initially denoted a specially cleared and enclosed area of land. In late antiquity, it came to be applied particularly to the sanctified land occupied by communities of Christian converts. It is part of the name of over 630 locations in Wales and nearly all have some connection with a local patron saint. These were usually (but not always) the founding saints of the parish, relatives of the ruling families who invaded Wales during the early Middle Ages. The founder of a new llan was obligated to reside at the site and to eat only once a day, each time taking a bit of bread and an egg and drinking only water and milk. This lasted for forty days, Sundays excepted, after which the land was considered sanctified forever. The typical llan employed or erected a circular or oval embankment with a protective stockade, surrounded by wood or stone huts. Unlike Saxon practice, these establishments were not chapels for the local lords but almost separate tribes, initially some distance away from the secular community. Over time, however, it became common for prosperous communities to either become monasteries forbidden to lay residents or to become fully secular communities controlled by the local lord.
In the later Middle Ages, llan also came to denote entire parishes, both as an ecclesiastical region and as a subdivision of a commote or hundred.