Lady Macbeth speaks about the unwashable blood on her hands, the striking of the clock--suggesting that time is short for the reign of her husband as well as her own life--and her husband's reluctance to perform the very murders that she so regrets now.
The suggestion is that Lady Macbeth is too guilt-ridden to maintain silence, though when awake, she forces herself to do just that. Shakespeare well knew that conflicts have a way of working themselves out, or at least atempting to resolve themselves, when one is asleep. Her unconscious mind is extremely conflicted. It is also interesting to note that Shakespeare has Lady Macbeth speak in prose, which the critic AC Bradley noted is the way that Shakespeare often wrote for had characters in abnormal states of mind.
Freud said that there are no accidents; perhaps Lady Macbeth wanted to be caught. We never find out, however, She is dead soon after this scene.
One of the more fanous lines from Shakespeare occurs here: Out, out.... This is echoed by Macbeth later, and of course, used as the title of a Frost poem.