Macduff tells Malcolm that things are not going well in Scotland with Macbeth in charge.
Malcolm says that they are going to weep over Scotland, and Macduff tells him there are problems in Scotland.
Let us rather
Hold fast the mortal sword, and like good men
Bestride our downfall'n birthdom. Each new morn(5)
New widows howl, new orphans cry, new sorrows
Strike heaven on the face, that it resounds
As if it felt with Scotland and yell'd out
Like syllable of dolor. (Act 4, Scene 3)
Basically, he is colorfully describing how the country is going to the dogs. Apparently with Macbeth in charge and the influence of the witches, there are a lot of supernatural occurrences that are just terrible. We have moved from horses devouring each other to constant crying over the state of the country.
Macduff is a noble and loyal countryman. He supports Malcolm and his country. He is not interested only in revenge, even after he learns about his family's murder. He wants to help Malcolm return to his rightful place at the throne to prevent the bloody tyrant from destroying the country further. Ultimately, he is one of the reaons they are successful, because he is the one who kills Macbeth.