Banquo says:
"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."
This means:
* "My thought": Banquo is referring to the witches' prophecy that he will be the father of kings, even though he hasn't done anything to deserve it.
* "Whose murder yet is but fantastical": He's worried that he might be murdered, just as the witches suggested Macbeth might be king, despite not being in line for the throne.
* "Shakes so my single state of man, that function Is smothered in surmise": This is a powerful image. The weight of the witches' words is so heavy that it's affecting his ability to think rationally. He's consumed by suspicion and worry.
* "and nothing is But what is not": He's questioning reality. He feels everything is uncertain and the witches' words are making him doubt what he knows to be true.
This is a significant moment in the play as it foreshadows Banquo's later murder and the darkness and instability that begins to consume Macbeth.