Macbeth was not worried about what would happen.
A figure of speech is no-literal language, or figurative language. It can be a simile, a metaphor, or an idiom. If it is an idiom, it would mean that it was a commonly used phrase. The expression used by the captain to describe Macbeth’s actions might be best described as a metaphor or personification. Personification is giving human traits to something nonliving. In this case, fortune is described as something that you fight or ignore.
Macbeth was a brave solider before the witches got hold of him. Evidence of this can be found in the captain’s descriptions of Macbeth’s actions during the battle that preceded the play. It appears that Macbeth knew no fear, and charged into battle until he defeated the traitor Macdonwald.
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,
Like valour's minion carved out his passage
Till he faced the slave … (Act 1, Scene 2)
In other words, Macbeth had to defy fortune in order to accomplish his task. This is because there were a lot of enemy soldiers he had to defeat before he could get to Macdonwald. Macbeth did not care though. He laughs in the face of fortune, so to speak. (That’s another figure of speech.) Macbeth had luck on his side.
This is actually a very important scene. It is easy to forget that Macbeth was once regarded as a valiant solider and a good man once the play gets started. Macbeth makes a lot of terrible choices, and is presented throughout as either aggressively ambitious or just plain insane. However, aggression and ambition are celebrated on the battlefield. Macbeth was a hero once.