There are paradoxes aplenty in Macbeth and this is simply one of them. The day on which the play begins is both good and bad. Macbeth has vanquished the king's enemies on the field of battle and been generously rewarded by Duncan for his extraordinary feats of valor. In that sense it's a good day. The newly ennobled Thane of Cawdor is a man on the make, rising rapidly through the upper echelons of the Scottish court.
Yet as Macbeth and Banquo arrive to see the three witches, they are treated to a stormy scene—a howling gale and a blackened sky whipped up by the wizened crones' dark, demonic powers. This is what Macbeth means by "foul." Unbeknown to Macbeth, his very first words in the play eerily echo those of the witches in act 1, scene 1:
Fair is foul, and foul is fair.
These words, like Macbeth's own, embody a paradox. Their similarity allows us to see straight away that Macbeth is inextricably linked with the forces of darkness and evil.