In the poem, the speaker is looking out the window at Dover Beach by night. The title and first stanza of the poem describe this setting. It is a calm night. The coast of France is visible across the channel; the famous white cliffs of Dover are "glimmering and vast;" the scene is "tranquil." A reference to "the moon-blanched land" lets us know that the moon is full or nearly full.
This is a time and place that would fill most people with a sense of well-being. They might find it calm, beautiful, or romantic. But surprisingly, that is not what the speaker takes from this particular time and place. Instead, it reminds him of "the turgid ebb and flow/Of human misery." By the end of the poem, after thinking about the desolateness of the human experience, he has almost descended into nihilism.
Apart from the obvious inference that the speaker brings some of his own despairing mood to the beautiful scene, the thing that seems to set his thoughts on their dark path is the "grating roar/Of pebbles which the waves draw back and fling,/At their return, up the high strand." Apparently he is close enough to the beach to hear this sound. If he were only able to see the beautiful scene, perhaps it would give a peaceful impression. But the sound of the pebbles, which is jarring, somewhat irregular, and relentless, is a rude reminder of the realities of a beach (the sea is rough with things and people), and so, of the realities of life. It may be a case of a beautiful scene being ruined by a little too much detail.