Fear not: an antecedent is nothing more than a word (or words) that come before a pronoun. The pronoun refers to that word, the antecedent.
You know what you mean, but your reader doesn’t. Problems arise when we use pronouns, but the antecedent either is unclear or missing. Here are some examples:
1. Robert smiled fondly at his brother and said he had saved his life. (Who saved whose life?)
2. Annie told Robin she was confused. (Who’s confused?)
3. Aaron is a good cook, which he practices daily. (What does he practice daily? The missing antecedent is “cooking.”)
4. Rosalie threw her iPhone on the tile floor and cracked it. (She cracked the tile or her phone?)
When you use a pronoun, picture drawing an arrow from that pronoun to the word it refers to. If your arrow goes nowhere, rewrite your sentence to clarify the antecedent.