In this passage, Macbeth is expressing stunned uncertainty.  He has just heard the prophecies of the Weird Sisters, followed by Ross's news that Macbeth has been promoted to Thane of Cawdor.  Immediately afterward, Macbeth has an aside in which he ponders the nature of the news he has been given, both by the witches and Ross.  The news initially came from the witches, which should make it an evil omen, yet it is good news.  Further, they also told him he would be king, which sounds great, but the beloved Duncan, the man who just promoted Macbeth, is currently king.  For Macbeth to be king, Duncan would have to die.  Because of the binary nature of the messages and outcomes, Macbeth doesn't know how to feel.  In addition, his ambition has already caused him to begin to think about what would need to be done in order for him to become king, which in turn causes him to question his own nature, as is demonstrated by the quote that you asked about:

My thought, whose murder yet is but fantastical,
Shakes so my single state of man
That function is smothered in surmise,
And nothing is but what is not. (1.3, 142-145)

Here Macbeth begins to think about the possibility of regicide, yet even the simple idea of it shakes him to his core.  His "function," or ability to act on the idea of murder, is hindered by all the thoughts of what could happen if he were to act.  He reaches the point of declaring that the only thing that "is," meaning "is relevant," is that which does not exist.  His ambition is causing him to covet the kingship, but his conscience is getting in the way of committing the act that will ensure it.  This uncertainty is the very same aspect of Macbeth's character that Lady Macbeth fears, and why she prays to be "unsexed" so that she may act more directly and definitely than Macbeth is able to.

Macbeth is expressing fear and doubt, which are very human emotions, and is aware of the contradictions in the initial prophecies.

Macbeth comments that the involvement of the witches cannot be good and cannot be bad.  This demonstrates doubt and confusion on his part, because he is not sure what is happening and how to feel about it. 

This supernatural soliciting

Cannot be ill, cannot be good. If ill,

Why hath it given me earnest of success,

Commencing in a truth? (Act 1, Scene 3)

This demonstrates the very human reaction Macbeth has to the witches’ prophecies.  He is not sure what to think, but he is concerned.  At the same time, he is pleased to have the opportunity to be king, and not too apt to question it.

This demonstrates that at this early point Macbeth is still very human, and still has typical concerns and confusion.