Remember, in literature, a foil is a character who contrasts another character through the character's qualities and characteristics.
Banquo is Macbeth's foil because his reactions to the witches premonitions are the opposite of Macbeth's. Through his reactions, we see what a good and loyal subject of King Duncan Banquo is. This leads the audience to realize Macbeth's reactions are not good and honest and have already led him to think about the possibilities for power he may have, if their predictions come true.
Banquo greets the witches asking for his fortune after they've given such noble titles on Macbeth, but instead of asking for power, he seems more interested in his future.
Good sir, why do you start, and seem to fear
Things that do sound so fair? I’ the name of truth,(55)
Are ye fantastical or that indeed
Which outwardly ye show? My noble partner
You greet with present grace and great prediction
Of noble having and of royal hope,
That he seems rapt withal. To me you speak not.(60)
If you can look into the seeds of time,
And say which grain will grow and which will not,
Speak then to me, who neither beg nor fear
Your favors nor your hate.
In Act 1 scene 4, Macbeth has already begun to think beyond his new title, Thane of Cawdor, at the possibility of being king. When Duncan names his son heir to the throne and Prince of Cumberland, instead of thinking about what he has, he begins planing what he needs to do to achieve the next step.
The Prince of Cumberland! That is a step
On which I must fall down or else o’erleap
For in my way it lies. Stars, hide your fires;
Let no light see my black and deep desires.