What is antecedent moisture?

An antecedent is the noun or pronoun that a pronoun replaces.

The pronoun that replaces the noun 'moisture' is it. Example:

The moisture is making my hair frizz. It also makes my clothes feel damp.

The Word 'it' is the pronoun, the Word 'moisture' is the antecedent.

The Word 'it' replaces the noun 'moisture' in the second sentence.