In this dramatic monologue, the speaker is addressing his wife or lover. He notes the cadence of the ocean's waves, bringing "the eternal note of sadness in." The repeating ebb and flow of the waves symbolizes the passage of time. The speaker ends this first stanza in a melancholy state as if to say the passage of time has led to this melancholy moment. In the next stanza, the speaker mentions Sophocles, an ancient Greek master of the form of play known as tragedy.
The speaker sets up this poem as an indication that something tragic has occurred. In the third stanza, he supplies an answer. The world was once comforted by the meaning, significance, and sense of religion. The "Sea of Faith" used to give people a way of understanding the world. But that sea is now ebbing (retreating) instead of flowing. With new theories in science and anthropology, the ideas of faith have less resonance with a modernizing world.
Without the comfort of faith, the speaker retreats to the only thing that can possibly provide comfort: romantic love. The world is new and unfamiliar. There seems to be hope in love but that love must exist in he and his lover. He does not see it in the world itself:
So various, so beautiful, so new,Hath really neither joy, nor love, nor light,Nor certitude, nor peace, nor help for pain;