The best thing to do for you to gain a good level of knowledge of this poem is to go through it with a dictionary and try and paraphrase it yourself. This will really help you to understand it in a way that merely reading my response will not. But, to get you started and hopefully to encourage you to visit this excellent poem, consider my humble efforts to paraphrase this poem.
One day in the middle of a harsh winter, I leant upon a gate in the countryside. Branches and twigs covered the sky, making them look like the strings of broken harps. The scene was so desolate, and I was alone, as every other person had gone inside to seek the warmth of their fires.
The bleakness and sharpness of the countryside seemed to represent the death of the century that is coming to pass, with the wind representing a kind of mournful dirge for the death of the century. In the winter, any traces of spring and new birth and growth seemed absent, and I found my feelings echoing this desolate sight.
However, the song of a thrush seemed to fill the scene with joy. Even though the thrush was old and frail, he seemed to brighten the atmosphere. This song stood in such contrast to the setting that it gave me an element of hope which he must have known something of, but I remained unaware about.