An example of foreshadowing is the story in the Prologue that the Moonstone is cursed.

Foreshadowing is a hint at something that is going to happen later in the story.

The prologue not only explains the Moonstone’s rich and significant history, it also explains how it became cursed and how the guardian priests watch it.

[The] Moonstone passed (carrying its curse with it) from one lawless Mohammedan hand to another; and still, through all chances and changes, the successors of the three guardian priests kept their watch…

These events turn out to be important, because the Moonstone does cause chaos in the genteel English family in the story.  Also, Victorians were highly interested in India and stories like this.  Setting up the mystery by describing this story makes it clear that something spooky is going to happen, and that there is something mysterious about the diamond—and trouble comes to whoever owns it or carries it.