In this first speech from Act I, Scene ii, fromMacbeth, the Captain or Sergeant describes the battle Macbeth has just won. He begins by saying the odds were against them because Macdonwald had many mercenaries (villainies), foot soldiers (kerns), and armed horsemen (gallowglasses). The Captain depicts a scene in which Macbeth and his soldiers were outnumbered. Thus, the odds were in Macdonwald's favor. "Fortune" can be defined as luck, chance, or some external factor affecting human affairs (such as fate, luck, chance, or the supernatural). In this battle, "fortune" applies to luck and odds. Luck and the odds were against Macbeth but he overcame those odds.
And fortune, on his damned quarrel smiling,
Show'd like a rebel's whore, but all's too weak;
For brave Macbeth—well he deserves that name—
Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution,(20)
Like valor's minion carved out his passage
Till he faced the slave,
Which ne'er shook hands, nor bade farewell to him,
Till he unseam'd him from the nave to the chaps,
And fix'd his head upon our battlements. (I.ii.16-25)
Fortune smiled on Macdonwald, giving advantage to Macdonwald. But this advantage (fortune) was not enough and Macbeth fought his way through the soldiers and killed Macdonwald, claiming his head as a trophy. Although "fortune" pertains to chance and 'the odds' in this passage, it does suggest, in the greater context of the play, how Macbeth overcame fortune (as fate or chance) in this early battle when he was still a virtuous, loyal soldier. But when fortune (as fate in the form of suggestions of, and prodding by, the witches) encourages him to do evil, he succumbs to it. Macbeth is an honorable champion in overcoming his lack of fortune but he becomes an evil murder in submitting to the prospect of future fortune.
In other words, it is honorable to fight through adversity (lack of fortune) and it is destructive to selfishly pursue fortune.