Can anyone explain this Macbeth quote? "Be this the whetstone of your sword..."?

Macduff's family have just been murdered on the order of Macbeth. Malcolm is telling him to let his anger drive him on to seek revenge on Macbeth. A whetstone is what you use to sharpen a sword, hence "be this the whetstone of your sword", means let your anger sharpen your readiness for battle. Don't blunt your emotions by giving in to grief.