The Doors of Durin, Lord of Moria. Speak "friend" and enter.

I, Narvi, made them. Celebrimbor of Hollin drew these signs.

╔══════════════════════════════════════════════════════════╗
║                                                          ║
║                    THE DOORS OF DURIN                     ║
║                                                          ║
║                                                          ║
║            ╔════════════════════════════════╗            ║
║            ║            ⚒️  👑              ║            ║
║            ║           ★★★★☆★              ║            ║
║            ║          🌳🌙    🌳🌙            ║            ║
║            ║               ⭐               ║            ║
║            ║                                ║            ║
║            ║   Ennyn Durin Aran Moria       ║            ║
║            ║                                ║            ║
║            ║     Pedo Mellon a Minno        ║            ║
║            ║                                ║            ║
║            ║   Im Narvi hain echant         ║            ║
║            ║   Celebrimbor o Eregion        ║            ║
║            ║   teithant i thiw hin          ║            ║
║            ║                                ║            ║
║            ║           C        N            ║            ║
║            ║              D                 ║            ║
║            ╚════════════════════════════════╝            ║
║                                                          ║
║                                                          ║
║               Speak "Friend" and Enter                   ║
║                                                          ║
║                                                          ║
╚══════════════════════════════════════════════════════════╝

The password is written in silver upon the stone.

What does it mean by "speak, friend, and enter"?