Before the scene opens in Act 4 Scene 2, Ross has told Lady Macduff that her husband has fled from Scotland. The scene opens with Lady Macduff being deeply agitated. She is demanding to know why he fled and is saying that fear may make a person flee and fleeing makes such a person look like a traitor even though they are not. She goes on to say that if Macduff was in danger, then surely she and her children must also be in danger.
Ross explains that it is either fear or wisdom that may make a man flee and that he believes Macduff to be wise and judicious. He also says that Macduff knows how to act wisely in dangerous times and eludes to the fact that Macbeth's spies are everywhere.
Ross ends by saying that if Macduff did flee because he was afraid, then he would have had good reason for his fear. Ross departs after saying he will return and on his way out sees Mcduff's boy who has heard Lady Macduff's ranting about MacDuff acting like a traitor.
To find the answer to this question, take a look at Act IV, Scene II. When this scene opens, Lady Macduff has learned of her husband's departure and her reaction is one of disbelief and disagreement. She says, for example, that his departure is "madness" and that he looks like a "traitor" because he has run away.
In response, Ross urges Lady Macduff to reconsider her position. After all, she does not know what exactly prompted him to leave:
You know not
Whether it was his wisdom or his fear.
In other words, Ross says that Lady Macduff should not jump to conclusions about her husband. Moreover, he tells her to pull herself together ("I pray you school yourself") and to remember that these are uncertain times when people are labeled traitors, even though they do not know what crime they have committed.
In essence, then, Ross implores Lady Macduff to support her husband and to understand that Macbeth is a dangerous and unpredictable foe.