Enotes is a great resource when you are trying to understand poetry, and I included some links below that give a detailed, line-by-line summary and explanation of the poem. That should really help.
To get you started on this poem, the lines above are the opening lines of Matthew Arnold's famous poem, "Dover Beach." In these lines, Arnold is simply using imagery (using the 5 sense to describe a scene) to paint the picture of the scene that he can see outside of his window while he writes this poem. He describes a sea that is calm and not super loud, and not having huge, crashing waves. The sea is quiet and calm that night. Then, he describes how the tide, or the ocean water, is all the way up on the beach (the tide is "full" means that the ocean water has crept up onto the beach pretty far--that's called the tide). Then, he mentions the moon that is shining upon the beach and the land that leads up to the beach, and the water. All of this helps the reader to imagine a calm, beautiful, tranquil evening where the moon is shining on a serene and full ocean below. This scene of tranquility is a set up, because as Arnold continues his poem, he is going to discuss the tumultuous events of human creation--war, greed, despair and misery. Starting the poem with such a calm scene is a perfect contrast to the chaos and misery that Arnold discusses. Nature is beautiful and perfect, as he describes in this scene, but mankind is chaotic, cruel and unnatural in their brutality towards one another. Using a peaceful nature scene at the begining of the poem makes that contrast more stark later on.
I hope that those thoughts help; good luck!