Reported speech is a grammar structure used to report what someone else said in the past by changing the verb tenses and pronouns. Key changes include changing the verb tenses to past tense, changing pronouns like "I" to "she" or "he", and changing time and place words. Examples show how direct quotes become reported speech by making these pronoun, tense, time, and place changes to indicate the statement is being reported after it was said rather than being said directly.