Is this a dagger which i see before me, the handle toward my hand? Come, let me clutch thee.Macbeth feels scared at suddenly seeing this apparition, and wants to hold the dagger to determine whether its real or not.
Or art thou but a dagger of the mind, a false creation, proceeding from the heat-oppressed brain? Macbeth is questioning why the dagger has appeared in front of him, and he thinks that it is because he is so stressed and under pressure - the heat-oppressed brain.
Thou marshall'st me the way that I was going; and such an instrument i was to use.Macbeth decides that the arrow is telling him to go and do the deed, it is pointing which way to go, and urging him to do it.
Mine eyes are made the fools o' the other senses.Macbeth is saying that his other senses are controlling his eyes, and telling them silly things.
And on thy blade and dudgeon gouts of blood, which was not so before.Macbeth feels even worse because the blade suddenly changes to one with blood and guts on.
There's no such thing: it is the bloody business which informs thus to mine eyes.Macbeth decides that it is just an apparition, and it is the deed that he is about to do that is making him see it.