The speaker of these words is Macbeth, who utters them in an aside,
Are less than horrible imaginings:
My thought, whose murder yet is but fantastical,
Shakes so my single state of man that function
Is smother'd in surmise, and nothing is
But what is not. (1.3.148-153)
Visibly shaken by the events of the day in which paradoxical elements exist--"So fair and foul a day I have seen"--Macbeth is confused by the equation of reality with fantasy. For, while he wishes to believe in the prophesies of the witches, he is shaken as he considers the fantastic idea that he may become king because in order for this to occur, the present king, Duncan, must die. If he does indeed become king, Macbeth realizes that something that is not at present true must take place. Therefore, what exists now will become "what is not."
In another aside, the exigent Macbeth hopes that chance will take care of accomplishing his becoming king and he will not have to do anything. But, he is "smother'd in surmise" as he deliberates, and is disturbed by his "fantastical" contemplation of regicide as a means of hurrying what is apparently destined to happen.
It is Macbeth who speaks these lines in Act I, Scene III, of the play. To put this quote into context, Macbeth has just spoken with the witches and they have prophesied that he will become the Thane of Cawdor and, more importantly, the King of Scotland. Now, Macbeth is musing on their predictions and thinking about how they might come about.
The first line of this phrase shows that Macbeth's thoughts turn immediately to murder:
My thought, whose murder yet is but fantastical.
This is interesting since the witches did not tell Macbeth that he would have to kill King Duncan. Moreover, it demonstrates the strength of Macbeth's inner ambition. He does not relish the idea of murder, however, and becomes very nervous ("shaken") about carrying out such an act:
Shakes my single state of man.
Taken together, then, these lines foreshadow Macbeth's murder of King Duncan and his rise to kingship.