Is a appositive the word or words a pronoun stands for?

No, an appositive is a Word renaming something before it in a sentence. An appositive can be a noun, a pronoun, or a noun phrase.

An appositive is always separated from the rest of the sentence by a comma or commas.

Examples:

His old dog, Spot, is always at his side. (the noun 'Spot' is the appositive renaming the noun 'dog')

The talented head chef, me, made the lasagna. (the pronoun 'me' is the appositive renaming the noun phrase 'the talented head chef')

Lucy, my sister, will pick me up today. (the noun phrase 'my sister' is the appositive renaming the noun 'Lucy')

A pronoun simply takes the place of a noun or a noun phrase in a sentence.

For example:

His old dog, Spot, is always at his side. OR, It is always at his side.

The talented head chef, me, made the lasagna. OR, I made the lasagna.

Lucy, my sister, will pick me up today. OR, She will pick me up today.