The scene is a deserted heath in Scotland. Three witches meet and discuss what they plan to do next. There is a battle going on in the distance, and thunder and lightning set the mood. The witches plan to appear to Macbeth, a general in King Duncans army. Then they leave the scene, telling us that Fair is foul, and foul is fair. This saying, which means, roughly, Things are not always what they appear to be, will take on more meaning as the play progresses.
A deserted spot. No one is around.
To come on stage.
Commotion. In this case, the battle.
Lost by one side, and (therefore) won by the other.
Desolate plain in Scotland.
Graymalkin is the name of the witchs familiar, or pet; in this case, a gray cat. A familiar was thought to be a lesser spirit attending its master in this case, the witch.
We can assume eerie sound effects were added from offstage during this scene (including the thunder specified in the stage directions); a cats meow, just before this line was spoken, would have been easy to produce.
Paddock is this witchs familiar, a toad. See notes on Graymalkin about familiars.
Someone backstage likely imitated a croaking sound to give the effect of a toad calling to its master.
The third witch also calls to her familiar. Anon means soon in this case, Ill be there soon.
Some other eerie animal sound effect was likely added here.
All three witches, speaking together.
Things are not always what they appear to be. Things that seem fair (virtuous, beautiful) on the surface will often turn out to be foul (vicious, evil, ugly), and vice versa.
Macbeth will learn, in the end, not to trust the appearances of the people and things around him. By then, however, it will be too late.
Latin, literally they leave. Everyone leaves the stage.