An antecedent is a phrase or word that the following pronoun refers back to. The antecedent makes the sentence clearer, as, without it, the subject remains vague with the use of pronouns. The antecedent and its pronoun usually agree in number and gender, meaning that if the antecedent is singular, the pronoun is also singular, and if the antecedent is plural, the pronoun is plural.
example: Othello by William Shakespeare
"Me thinks the wind has spoke aloud at land,
A fuller blast ne'er shook our battlements,
If it hath ruffianed so upon the sea
What ribs of oak, when mountains melt upon them"
(Wind is the antecedent the pronoun "it" refers to)