In William Shakespeare's Macbeth, what is the significance of Act I, Scene 1?

The first scene of Macbeth serves as a prologue to the
whole Play and Introduces in the form of the Witches (Weird Sisters, weird being a
derivative from the old english word "wyrd" which translates into "fate" or "destiny")
the forces of action that will control the fate of the different characters. This scene
serves also to set the general mood of the play and the setting of it, as well
as Shakespeare's use of images in his language (thunder and lightning) Each phrase and
word pronounced by the Witches will have its resonance further on in the next
scenes.

ACT ONE, Scene I.

An
open place. Thunder and lightning. Enter Three
Witches.

First Witch:

When
sall we three meet again, / in thunder, lightning or in
rain?

Second Witch:

When the
hurlyburly's done, / when the battle's lost and won.

(When
the hurlyburly's done: The witches propose to meet again when the battle now in progress
is finished - whe nit is lost and won --> lost by one side and won by the
other)

Third Witch:

That will
be ere the set of sun.

(ere the set of sun- "before
sunset")

First Witch:

Where
the place?

Second Witch:

Upon
the heath.

(They announce the place where Scene III will be
set)

Third Witch:

There to
meet Macbeth.

(Introducing the character of
Macbeth)

First Witch:

I come,
Graymalkin!

(Graymalkin is the name of a cat; witches were
said to keep cats as assistants in theri
ceremonis.

ALL:

Paddock calls.
- Anon! -/ Fair is foul, and foul is fair:/ Hover through the fog and the filthy air.
[exeunt

(Paddok - "toad/ Anon! - "(We are coming) at once"/
"fair is foul, and foul is fair" The good and bad are confused; there is an evil
influence. This particular verse will be of significance as the rest of the play is
unfolded./ "Hover though the fog and the filthy air" imagery used by Shakespeare, along
with thunder and lightning to set the place and the mood of the
scene.)

As you can see the Witches are useful instruments
for Shakespeare to announce what will happen in the play, to slip little pieces of
information into the audience: here we are told of a battle taking place that shall end
before the setting of the sun with one side winners and the other losers, and most
importantly, we are told of a future gathering with Macbeth, which shall take place in
Scene III and will be of crucial importance for the rest of the
play.