"Yet do I fear they nature;
It is too full o' th' milk of human kindness
To catch the nearest way."
Here she has heard about the prophecy, but she knows that he is a good kind person and won't take the throne by force. Shakespeare could also be suggesting that Lady Macbeth doesn't have this "milk of kindness."
"Hie thee hither,
That I may pour my spirits in thine ear
And chastise with the valor of my tongue
All that impedes thee from the golden round."
Lady Macbeth is explaining that she has the evil attributes that Macbeth needs to make himself king.
Any other questions, let me know! Thanks!
Teacher Charles