In act 5, scene 1, Lady Macbeth is shown sleepwalking, during which time she is fixated with the act of washing her hands, convinced that they are stained with blood. As she sleepwalks, she can also be observed speaking to herself (and it is through these words that Shakespeare allows us a glimpse into the state of her mind).
Ultimately, by act 5, scene 1, Lady Macbeth is shown overcome with guilt. She had previously assisted Macbeth (and even drove him on) in his murder of Duncan and usurpation of the throne. However, it seems that during this earlier part of the play, she had underestimated the degree to which these actions would later weigh on her conscience. Furthermore, note also that, in this scene, her guilt extends beyond her role in that initial murder of Duncan: thus, in her utterances, we observe Lady Macbeth making reference to the murder of Lady Macduff:
The thane of Fife had a wife: where is she now? (act 5, scene 1)
What we observe in this scene is a window into a tormented mind, overcome by guilt for her and her husband's actions. Macbeth, as a play, does have a psychological component to it, as it follows the psychological repercussions that the act of murder has on its perpetrators: while Macbeth is pushed further into bloodthirsty megalomania, Lady Macbeth is tormented by her guilt.