Antecedent is a noun in English language and its literal meaning is something that exists before a certain time or thing. In other words, when something precedes another, it is known as antecedent.
In English grammar, antecedent has somewhat similar meaning. A word or phrase is said to be antecedent if it gives its meaning to another word that is pro-form or a pronoun used afterwards in the text.
Samantha ate the whole pie all by herself.
In the above simple example, the noun Samantha is the antecedent of the pro-form herself because the pronoun herself used later in the sentence is referring to Samantha.
Explicitly, an antecedent is a word or phrase that is referred back to in a text by a relative pronoun or any other relative word in the same text.
She went back inside to get the umbrella which she had accidently left in the balcony.
In the above example, the umbrella is the antecedent of the relative pronoun which.