These are the opening lines of Lady Macbeth's entry speech, a soliloquy, in act 1 scene 5. The lines refer to what Macbeth has confided to his wife in the letter just read out by Lady Macbeth. Macbeth was the Thane of Glamis, and he has been crowned with the title of Cawdor by King Duncan as a reward for his exemplary performance in the battles against the rebels. Lady Macbeth endorses the proclamation of the witches that her husband shall also be the king. But she is afraid of some constraint which she believes to be inherent in Macbeth's nature: It is too full o' the milk of human kindness. This is a mistake on the part of a wife who does not possess enough insight into her husband's dilemma. She mistakes Macbeth's habit of shilly-shallying for kindness of disposition. If Macbeth is unable to settle down to "catch the nearest way" (an euphemism for murder), it is not because he is characteristically of a very sympathetic disposition. Further on here, Lady Macbeth under-estimates her husband's deep-seated ambition, and over-estimates his conscientious scruples about the means to that end:
.......................................thou wouldst be great;
Art not without ambition, but without
The illness should attend it: what thou wouldst highly,
That wouldst thou holily; wouldst not play false...