Your question is refering to Act I scene 2, which is when King Duncan is awaiting news of the battle that is being fought by his forces, and a wounded messenger arrives to give them a fresh update on the state of play. After the encounter with Macdonwald and his forces with Macbeth and his Scottish troops, the next issue that threatened Duncan's forces was a renewed assault by the King of Norway that had the potential to overwhelm the Scottish forces. Note how the messenger describes this event:
No sooner justice had, with valour arm'd,
Compell'd these skipping Kernes to trust their heels,
But the Norweyan Lord, surveying vantage,
With furbish'd arms, and new supplies of men,
Began a fresh assault.
However, in spite of the threat that this renewed vigour of the enemy represented, it was not enough to make Banquo and Macbeth afraid, and this vigour of the enemy only inspired them to strengthen their own attack, leading to their ultimate victory.