In his poem "Dover Beach," Matthew Arnold relates the feeling that he has that all beauty, faith and love is retreating from the world, leaving behind only misery, sadness, struggle and war. In the first stanza, he describes the beautiful scene from the window--the moon, the night air, the beach, the waves. But then, he describes the sound of the waves as showing "sadness" and "misery." In the third stanza, he describes how a "sea of faith," meaning, a world that is filled with faith, beauty and love, used to be the world that we lived in. But, that world is "retreating, to the breath of the night-wind." This is simply saying, poetically, that he feels that faith, love and beauty are retreating, or going away. The night-wind part of it just indicates that darkness is coming in its place, and beauty goes away as dark night comes in (it's just a poetic reference, tying nighttime to the dying of faith.) The next phrase, "down the vast edges drear" is Arnold attempting to paint a cold, distant, lonely image of the world. Faith is leaving the world, and departing from the vast edges of the world (large and distant), edges that are drear. Drear means that the world is dreary and miserable. Then, "the naked shingles of the world" is Arnold using a metaphor to describe how exposed, alone, and vulnerable the world is now that faith has left it. When love and faith disappear, it leaves the world naked, exposed to all of the elements of evil and misery, just like shingles are naked, exposed to the stormy sky.
I hope that those thoughts help; it's a beautifully written poem that uses great imagery and poetic techniques to convey a sense of loss, despair, and at the end, hope. Good luck!