In the first stanza of "Dover Beach," the speaker watches a lighthouse on the French coast. The light "Gleams and is gone" and the disappearance of the blinking light symbolizes a feeling of darkness and isolation. This loss of light can be interpreted as a metaphor for the loss of a guiding light in the world.
After describing the sound of suffering between the pebbles and the waves and as the speaker understands the thoughts of Sophocles in ancient Greece, he remarks that Christianity (the "Sea of Faith") once gave meaning to the world and ameliorated that suffering.
The Sea of Faith
Was once, too, at the full, and round earth's shore
Lay like the folds of a bright girdle furled.
The speaker, and Arnold himself, felt that the power of Christianity and religion had diminished; religion as a guiding light was fading. That is, religion was losing its ability to unify people and to give meaning especially during difficult times.
Since the speaker no longer has faith in the unifying power of religion, he seeks a substitute in his relationship with his lover. The final image is of he and his lover alone trying to make sense of the world.
And we are here as on a darkling plain
Swept with confused alarms of struggle and flight,
Where ignorant armies clash by night.