In the first scene of the play, the Weird Sisters are planning their next meeting, a meeting with Macbeth. In the first line, one sister asks, "When shall we three meet again?" (1.1.1). The second sister says, "When the hurly-burly's done, / When the battle's lost and won" (1.1.3-4). We find out, in the next scene, that Macbeth has been fighting in a battle, and that he and his forces have won, succeeding in putting down a civil rebellion as well as defeating the Norwegian king's army.
The sisters say that, after the battle is over, they plan to "meet with Macbeth" on the heath (1.1.8). A heath is a type of land, rather typical for Scotland, made of mostly rocky and uncultivated soil, somewhat barren except for hardy, weedy plants like thistle. Such a meeting cannot bode well for the titular character, as the creepy and sinister mood established by the setting as well as the Weird Sisters' speech patterns (the meter and end rhymes) alerts us to the fact that they are not "good" witches. Their final lines provide a clue as to the sisters' intentions. They say,
Fair is foul, and foul is fair;
Hover through the fog and filthy air. (1.1.12-13)
This makes it sound as though the witches are going to make good, "fair" things seem "foul" or bad, and "foul" or bad things seem "fair" and good. This seems to indicate that they plan to deceive Macbeth at this meeting.