When the play begins, the witches enter with thunder and lightning. A storm is coming. This already foreshadows something dark and ominous. The first witch asks when they will meet again, "In thunder, lightning, or in rain?" (1.1.2). In Shakespeare's time, witches were associated with Satan and evil in general. As part of the folklore about witches, it was also believed that they could cause bad weather. So, quite literally, they could change the landscape and the environment. In other words, they could cause things in the world to become dark and evil. They set the stage (pun intended) with a general sense of evil.
The first and second witches mention Grimalkin and Paddock. These are called "familiars" which are attending evil spirits. With the storm and the attending evil spirits, the witches are imbuing the world with darkness, storms, and evil. The scene ends with all three chanting:
Fair is foul, and foul is fair,
Hover through the fog and filthy air. (1.1.10-11)
Again we have the repetition that the environment has become filthy and evil. With these final lines, the witches indicate that what had been good ("fair") will become evil ("foul"). This foreshadows Macbeth's descent from a loyal Thane to a murderous tyrant. He was fair and will become foul. The witches symbolize this shifting in the world from foul to fair.
There is always debate as to whether they cause Macbeth's downfall or whether they simply plant the idea in his mind. The latter supports the idea that they put evil in the air and that they put greed and ambition in his mind, leaving it up to him to act upon these thoughts.
They foreshadow something dark and evil is about to occur. They also give insight to the supernaturalism in Macbeth.