Lady Macbeth fears that her husband's nature is too gentle and compassionate to "catch the nearest way" to the throne; by this she means the fastest way, which would be murdering the current king to usurp his role and position (1.5.18). She believes that Macbeth is "too full o' th' milk of human kindness," a metaphor that means caring for other people. Lady Macbeth is afraid that Macbeth does not have the ruthlessness and callous ambition needed to kill the king: a man who is his friend, his relative, and someone who trusts and values Macbeth implicitly.
Seen a bit differently, however, we might consider this expression to be an example of a different literary device. A metaphor compares two unalike things, saying that one thing is another, and certainly compassion and care are unlike milk. However, milk is often associated with motherhood and, therefore, with maternal feelings such as care and love and concern. Metonymy is a figure of speech in which something associated with another thing is substituted for that thing. So, here, "milk of human kindness" could be considered a substitution for care and compassion, and this would make the line an example of metonymy.
Either way, we soon see that Lady Macbeth herself wants to get rid of any potential she might have for "remorse" or "compunctious visitings of nature" that might prevent her from going ahead with the plan to kill Duncan (1.5.51, 1.5.52). She actually does pray to have evil spirits "Come to [her] woman's breasts / And take [her] milk for gall," another reference to milk being associated with gentle and loving feelings (1.5.54-1.5.55). She contrasts these feelings associated with breast milk with "direst cruelty" (1.5.50). For this reason, I think we can call the device used either a metaphor or a metonym.