The Witches are telling the future or and destiny of Macbeth who will commit treason by killing the king.
"Fair is Foul, and Foul is fair: Hover through the fog and filthy air.
Lady Macbeth wants her husband to look happy and innocent on the outside but be ready to strike bite like a snake when the time comes.
"Look like the innocent flower, but be the serpent under't."
Macbeth is seeing or hallucinates a dagger, and is about to kill the king.
"Is this dagger which I see before me, the handle towards my hand?"
Macbeth is hearing imaginary voices. He is feeling guilty for killing the king.
"Still it cries "Sleep no more!" Macbeth shall sleep no more.
"I fear thou play'ds most foully for't."
Banquo thinks that Macbeth did something bad to become king.
"I am in blood stepp'd in so far that, should I wade no more, returning were as tedious as go o'er."
Macbeth is saying that stepping in blood so deep by killing people that returning to where he started would be just as tedious or boring as moving forward in continuing to kill people.