In Act I, Scene 5 of Shakespeare's Macbeth, lines 20 and 21, Lady Macbeth says, "Thou'dst have, Great Glamis,/That which cries 'Thus thou must do,' if thou have it,..." In the speech that these lines are a part of, Lady Macbeth reveals that she knows Macbeth has the necessary ambition to desire the throne of Scotland, but that he may not be evil enough to do what's necessary to achieve it (kill the current king). In lines 20 and 21 she's saying that Macbeth needs a voice inside of himself that cries, paraphrasing, "You must do what's necessary, if you would have it (the thrown)." This voice, Lady Macbeth feels, might help Macbeth follow through and do what he wants to do (kill the king and claim the thrown).
It's not clear to me if you are just asking about those exact words, or the overall context they're in.
The lines you cite are in Act I, Scene V. Lady Macbeth has just received a letter from her husband in which he talks about the witches' prophecy and his desire to become king.
In the speech you cite, she is saying that Macbeth is not ruthless enough to really become king. The exact words you cite just mean "you want, Macbeth." But these words are a part of larger speech where she says Macbeth wants all sorts of things but doesn't have the ruthlessness needed to get them.