From Final Fantasy Wiki

The Goddess's Bell is a key item from Final Fantasy II. An ancient key, it is sealed deep in the Snow Cavern.

Story

After the Kashuan Keep was built, the King of Kashuan worried that people would try and steal it. So he built a door and three keys. Unless you were a member of the Kashuan royal family, it was impossible to open the door without a key.

The three keys were built from friendship, love, and wonder. The keys built from friendship and love soon broke, but the one built from wonder never did, and it was this key that became the Goddess's Bell. Pleased with this key, the king ordered that it should be hidden in a cavern of snow. It was, and it was guarded by an Adamantoise.

Eventually, Firion and the party come to retrieve the bell so that they may enter the Keep, since Gordon had gone missing.