This refers to Macbeth's valor in fighting against the "stout Norweyan ranks," the army from Norway that was contending with King Duncan of Scotland. Duncan (the "He" in the quote) is planning to reward Macbeth for his courage in defeating the Norwegians by granting Macbeth the title of Thane of Cawdor, thus fulfilling one of the witches' prophecies.

What Ross is saying to Macbeth, through indirect dialogue summarizing what Duncan said earlier, is that Duncan was enormously pleased with Macbeth's action on the field of battle in the fight against the Norwegian soldiers ("Norweyan ranks") and that every time Duncan saw Macbeth on the battlefield that same ("selfsame") day, he noted that Macbeth was deep in the fray of battle, not sparing himself by commanding from the sidelines. Ross tells Macbeth of how Duncan praised his valor and his actions.