What is one example of foreshadowing in Macbeth? What is the significance of foreshadowing to the play as a whole?

There are many examples of foreshadowing in
Macbeth. The witches' prophecy, the bloody battle in Act I, and the
murder of Duncan and its link to sleeplessness.

To answer
your question, perhaps one of the most important examples of foreshadowing happens
during the murder of Duncan.  When Macbeth murders Duncan, he
hears:

There's
one did laugh in 's sleep, and the other one cried
'Murder!'

This can be looked
at as foreshadowing of Macbeth's upcoming problem with sleep.  Macbeth even mentions to
Lady Macbeth:

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Macbeth will sleep no
more.

Macbeth knows that what
he has done is wrong, that the murder of Duncan will affect him in some way, and that he
has already begun to slip mentally. In the end, it is the murder of Duncan that leads to
Macbeth's inevitable slip into insanity and insomnia.