To provide more information or to identify a word, a noun or pronoun is placed with another noun or pronoun, referred to as an appositive phrase.
An appositive phrase may be nonessential or essential.
In essential appositive, the words are not set off with commas.
Whereas, nonessential appositive refers to the addition of meaning in a sentence which is already clear and is set off with commas.
Therefore, phrase which is an appositive in the above sentence is "Snowy Heat" as it qualifies the noun guitar composition.