This term comes from the Greek term for a giant fennel plant and refers to the area of the church that can be thought of, somewhat crudely, as the lobby (i.e., the place where you put the plants.)

In the early church the narthex was separated from the nave by a low wall or screen, and reserved for the catechumens (those preparing for Baptism) and penitents, who were not admitted amongst the congregation to receive communion until the Great Vigil of Easter, when they would be invited to receive the communion.

Fortunately, we no longer have such restrictions as even those who are not Baptized are welcomed into the nave to take their place with the rest of us, and may come to the table to receive a blessing.

Therefore the Narthex no longer serves to keep people out, but to invite in all who hunger and thirst for righteousness; although, for those of us who have ever been late for church knows, the narthex can seem like a safe place to "wait it out" until we can gracefully "slip in" to the pews.