The scene you want to focus on is Act III scene 6, in which Lennox talks with another Scottish Lord about his suspicions of Macbeth and the forces that are being raised in opposition to his rule. It is the Lord that tells Lennox about these forces. He says that Malcolm has gone to England, where he is being helped by King Edward to raise a force to retake his rightful inheritance that has been so cruelly snatched away from him by Macbeth. In addition, Macduff has gone there to gain more support for their cause. Note what he says:
Is gone to pray the holy King, upon his aid
To wake Northumberland, and warlike Siward;
That, by the help of these (with him above
To ratify the work), we may again
Give to our tables meat, sleep to our nights...
Therefore this scene is important in giving the audience news of the conflict that is in the offing between Malcolm and Macduff on the one hand and the forces of Macbeth on the other. This of course foreshadows Macbeth's eventual defeat and, ironically, the truth of the prophecies of the witches, for Macbeth will not have an heir that can inherit the crown from him, and his ambition will end with his death.