The speaker describes a farmhouse that has been overgrown with rust and moss. In other words, it's a dump. The walls and ceiling are crumbling, and the gate is rusted over. Sheds stand broken outside, and they look strange. There's also a moat, though this isn't quite a castle from a fairy tale.
Mariana is here, letting us know that her life is dreary because a man won't return to her, and she wishes she were dead. There's no prince coming to her rescue and she knows it—sniff. Mariana continues to cry all day and night and can't think of anything positive. When she looks out the window and sees a poplar tree standing alone, she is even more despondent.
All the while, Mariana repeats her refrain that ends in a wish for death. She especially hates sunset, because the light comes inside and reveals dust (apparently there were no vacuums back then) and her lack of activity. She's also unhappy when the shadow of the poplar falls across her bed.
Then it starts to get extra creepy. Within the house, the noises of clocks and mice begin to drive her insane, and Mariana sees and hears "ghosts" from her past. In this state, she finally decides that her man is never coming back. She ends by wishing, again, for her own death.
And that's where the poem ends. Cheery, eh?