Answer:
Ross knows that Macduff's wife and son have been murdered by orders of Macbeth. Ross has to deliver these news to Macduff and he doesn't want him to hate him forever after the he tells him what has happened.
This is located in Act 4 Scene 3 Lines 203-204 of the Tragedy of Macbeth.
Explanation:
Macduff doesn't know this yet, this is why Ross says:
"Let not your ears despise my tongue forever" = Don't hate me forever for the words that I'm about to say.
"Which shall possess them with the heaviest sound that ever yet they heard" = My mouth is going to voice the saddest words that you will ever hear.
What comes after that is the terrible news for Macduff. His son and wife were killed and he was not at home to defend them.
