Well, this great poem isn't that long so I would really encourage you to read it for yourself - it is an unforgettable and in many ways haunting tale that lingers in the mind long after you have finished it. I have included the link to the enotes summary below and hopefully my explanation will help you make sense of it. Persevere! It is worth it.
Parts I and II the speaker sets the scene and introduces the plot. The Lady, on her remote island, is under a curse: She must keep to her weaving and ignore Camelot. However, it is clear that she is attracted by the reflections of the world she sees in her mirror. At the sight of two lovers, she declares that she is "half sick of shadows." In Part III, the Lady glimpses the dazzling figure of Sir Lancelot as he rides to Camelot. She leaves her loom and looks toward Camelot, whereupon the mirror cracks and the lady recognises that she has brought the curse down upon herself. In Part IV, she then goes down to the river, writes her name on the prow of a boat, and floats toward Camelot, singing. By the time she reaches the first house, she is dead. The people of the town gaze in fear and wonder at her lifeless form, while Lancelot utters a prayer.