When Ross enters the scene, he asks Macduff not to judge him for the terrible news he bears: the fact that Macduff's wife and children have all been slaughtered. Naturally, Macduff is devastated by this news, but Malcolm suggests that they should make "medicines of our great revenge." What he is saying here is that they should use their grief and anger in order to fuel a "medicine," or a cure, for that grief: by avenging the deaths of Macduff's family, they will be using their feelings of sorrow to drive them towards an act which will ultimately make them feel better. Grief should not "blunt" Macduff's sword, but "enrage" it, making it sharper. More harshly, Malcolm tells Macduff to "dispute it like a man," or rather, respond with violence and firmness to the blow that has been dealt rather than allow himself to be overwhelmed with grief.

Macduff turns this idea on its head by stating that, while he will respond to Macbeth with force, he will also "feel it as a man." A man, Macduff is saying here, is not only strong and violent, but also a person with feelings and sentiment. This questioning of gender roles is an echo of Lady Macbeth's earlier request to be "unsex[ed]" or freed of sentimental feelings typically associated with womanhood.

Building on the last reply, this idea of "take it like a man" is repeated throughout the play.  Lady Macbeth first goads Macbeth into killing Duncan by suggesting that he is less than a man.  Lady Macduff, in her conversation with her son, also insults her husband's manliness.  Macduff, the "hero" at the end, is the only character to routinely exhibit elements of compassion.  Thematically, Shakespeare appears to be criticizing behavior that is overly "macho".  As he was writing this shortly after King James ascent to power in England, he might have been giving credit to the better leadership provided by a woman, in the form of Queen Elizabeth.