In the play Macbeth, Malcolm says, " Angels are bright still, though the brightest fell. Though all things foul would wear the brows of grace, Yet grace must still look so." What does that mean?

It means that things usually aren't what they seem to be. 'The brightest fell' - Lucifer was the brightest angel, yet he fell to hell and became quite the opposite - the Devil himself. And nobody expected that would ever happen, but it did. The same way nobody expected Macbeth to be what he turned out to be.