What does this quote mean and symbolize in Macbeth? Is this a dagger which I see before me . . . . To feeling as to sight . . . .

The passage reads as follows:

Is this a dagger which I see before me,

The handle toward my hand? Come, let me clutch thee.

I have thee not, and yet I see thee still.

Art thou not, fatal vision, sensible

To feeling as to sight . . . .

Macbeth witnesses this dagger, floating in the air before him, shortly before he goes into King Duncan's chamber to murder him while he sleeps. When Macbeth sees the dagger, he speaks to it directly and tries to grab it, but cannot because it is not a physical reality. This astounds him; he can't touch it, but he continues to see it. He speaks directly to the dagger again, addressing it as "fatal vision," trying to determine what exactly it is, if it is something he can touch as well as see, or if it is something else. The passage continues with Macbeth's wondering if the dagger is only "a false creation" of his disturbed mind. 

The appearance of the dagger, which turns bloody before Macbeth's eyes as he continues to watch it, adds another eerie supernatural element to the play. Symbolically, it represents Macbeth's inner turmoil and feelings of horror about the murder he is about to commit, just as the appearance of Banquo's ghost in Act III shows Macbeth's fear and guilt for the most recent murder he has committed at that point in the plot.