What's an Antecedent?
An antecedent is a word, phrase or noun to which a pro- form such as a pronoun refers to. An antecedent serves to give meaning to a pro-form.
A pro-form often follows its antecedent
Here's an example
The Dwarf who dwelled in the mountains, lost his magic axe.
In this sentence the antecedent is Dwarf , it adds meaning to the pro-noun "who".
The witch appeared by magic to let us know she was running late.
In this sentence the antecedent is witch and the pronoun it refers to is she.