There is no real equivalent in English to the Celtic morpheme (word part) whose Welsh version is Llan. It's commonly said to mean a church, but one thing it does not mean is a church building. The Welsh word for that is eglwys (pronounced somewhere between egg–lewis and egg–lois).
According to Wikipedia, "The various forms of the word [llan] are distantly 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 ... Unlike Saxon practice, these establishments were not chapels for the local lords but almost separate tribes, initially some distance away from the secular community ... 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."
In summary: a llan was originally a piece of land that was sanctified for use by a Christian community, on which a church may well have been built; it later came to signify a parish.
But if you're asked in a quiz, what the Welsh word llan means, your best bet is probably to answer "a church".
© Haydn Thompson 2022