This is actually a highly significant point in the play. Let us just remind ourselves what has happened. Macbeth, having killed Duncan, and Lady Macbeth, having framed the two servants by planting the knives and the blood on them, are innocently waiting for the crime to be discovered. When the alarm is raised, Macbeth rushes in, and then kills the two servants. As he exits and tells those present what he did, he faces an incredibly suspicious question from Macduff, asking him why on earth he killed the two men who could give them information as to why and how Duncan was killed. Of course, now that Macbeth has killed them, he has "confirmed" that it was they who did the crime, as they can now not defend themselves.

It is at this stage that Macbeth has to think on his feet very quickly and come up with some kind of reason or excuse for his hastly slaying of these two servants. The quote you have highlighted comes as part of this, which shows Macbeth desperately struggling and making an unconvincing speech, laced with hyperbole and figurative langauge, to explain his actions. He does such a bad job, that Lady Macbeth feels that she needs to create a diversion by fainting, therefore taking the attention off her husband and distracting Macduff from his pathetic excuse.