A metochion is essentially an ecclesiastical embassy, usually from one autocephalous church to another, but occasionally the term is used to refer to the parish representation of a monastery in a nearby town.
In the former case, the territorial autocephalous church grants a plot of land or a church building for the use of the foreign church being represented, and the location is then considered to belong to the foreign church. Services held there are often in the language appropriate to the church being represented, and the congregation is often made up of immigrants or visitors from the nation associated with that church. Typically, a metochion presence on the territory of an autocephalous church is limited to only a few parishes at most.
In the case of a monastic metochion, such a parish church is a dependency on a particular monastic community, perhaps receiving clergy from that community or other forms of support.