An appositive is a noun or a noun phrase that renames another noun right beside it. It can be short or long.
On this case, the appositive is nonessential because its surrounded by commas, separated from the rest of the sentence. It interrupts the sentence.
If the information given in the appositive is essential to the sentence´s meaning is called Appositive restrictive and commas are no longer used for these cases.
The alchemist says that the wind told him Santiago was coming and would need help. He instructs Santiago to sleep well, trade his camel for a horse, and remember that his treasure will be where his heart is.
