The original meaning of 'Llan' was an enclosed piece of land within which would be a church and a group of Christians. Each 'Llan' or 'sacred place' was established by a Celtic saint after whom the site was named.
It is often transcribed as 'the Church of...' but would more accurately be described by 'parish'.
The Breton and Cornich equivalent is Lan as in Lan-stefan-ton = Launceston. The part of the place name immediately after Llan is usually the name of the saint to whom the parish was dedicated - e.g. Lampeter - St Peter, Llanfair - Mary, Llantrisant - three saints,