Malcolm and Macduff, in Act 4, scene 3, have just learned that Macbeth had Macduff's wife and children killed. Macduff is stunned almost to being speechless. Malcolm tells Macduff to let the grief Macduff is feeling now be the cornerstone of their desire for revenge against Macbeth. Malcolm says, "Be comforted: / Let's make us medicines of our great revenge, / To cure this deadly grief."
A few lines later, when Macduff asks that Heaven give rest to his slain family and Malcolm says, "Be this the whetstone of your sword: let grief / Convert to anger; blunt not the heart, enrage it." Malcolm is telling Macduff to not let his grief disable him but to let it enable him because they will go to Scotland now, along with help from the English, to overthrow Macbeth.