Matthew Arnold's "Dover Beach" is an expression of concern and existential disquiet about the state of faith in Victorian England. At that time, ideas were changing quickly, and the speaker is afraid that the "Sea of Faith" is retreating from the island, leaving the people unsure of where to turn or what to believe. It is in this spirit that he compares himself and his readers—expressed collectively as "we"—to "ignorant armies" clashing with each other in a state of confusion.
Arnold famously describes these people, himself included, as being stranded on a "darkling plain." The darkness here is metaphorical as well as literal. The people are no longer able to see by the clear lights of the faith they had once known; instead they find themselves in the dark, "ignorant" of what is the right path and clashing uselessly with each other, both groups effectively blind. Like Sophocles long ago, struggling to find meaning in confusion, the speaker hears a "note of misery" in the sound of the sea because he cannot identify what is true any more.