An appositive is a noun, a noun phrase or a noun clause that comes next to another noun that renames it or describes it in another way. The appositive can be identified because it can be taken out without a loss of meaning because it gives extra information about the renamed noun or noun phrase
I kept hearing that horse, the one with the white blaze, whinnying loudly
If the sentence is "I kept hearing that horse whinnying loudly" without the appositive would be completely comprehensible
