The narthex of a church is the entrance or lobby area, located at the end of the nave

Nave

In Romanesque and Gothic Christian abbey, cathedral basilica and church architecture, the nave is the central approach to the high altar, the main body of the church. "Nave" was probably suggested by the keel shape of its vaulting...

, at the far end from the church's main altar

Altar

An altar is any structure upon which offerings such as sacrifices are made for religious purposes. Altars are usually found at shrines, and they can be located in temples, churches and other places of worship...

. Traditionally the narthex was a part of the church building, but was not considered part of the church proper. It was either an indoor area separated from the nave by a screen or rail, or an external structure such as a porch.
Traditionally the word comes from narthex (Medieval Latin from Classical Greek narthex νάρθηξ 1.