How does Shakespeare develop the motif of blood over the course of the play? I don't really get the question . . . I know that it helps the audience understand the main theme of guilt and all, but I'm a bit confused. I would really appreciate it if someone would at least help me understand the question a bit or answer it, thank you.

Early in the play, beginning in scene two of the first act, blood is associated with military valor and honor. The sergeant approaching Duncan is described simply as "that bloody man." When the sergeant describes Macbeth's valor in the battle to preserve Duncan's throne, he evokes the image of a sword steaming with blood in the midst of the fighting. Once Macbeth murders the king, however, blood begins to connote his guilt. Even as he is making his way into the king's chamber, he sees a vision of a bloody dagger and interprets it as having been conjured by the prospect of killing the monarch. After the murder is carried out, Lady Macbeth tells him to wash the "filthy witness" (the king's blood) from his hands.
Once Macbeth kills Banquo, he sees another vision. This time Macbeth sees his murdered friend, and Macbeth beseeches Banquo not to shake his "gory [bloody] locks" at him. This he interprets (though his wife disagrees) as guilt, and one can see how the motif of blood has been developed. The connection between blood and guilt can best be seen in Lady Macbeth's famous sleepwalking scene in act 5, scene 1, in which she tries in vain to scrub imaginary spots of blood from her hands. In this scene, the connection between blood and guilt is explicit, and it is clear that Lady Macbeth has been consumed by a guilty conscience.