Elle est allée au marché.
English: She went to the market.
In French, the past participle (participe passé) changes in some cases to agree with the subject or object. This is called 'agreement of the past participle' and depends on whether the auxiliary verb is 'être' or 'avoir'.
You use the agreement of the past participle in French when forming compound tenses (like passé composé). With 'être', the past participle agrees in gender and number with the subject. With 'avoir', the past participle usually does not change, unless the direct object comes before the verb.
Elle est allée au marché.
English: She went to the market.
Nous sommes partis tôt.
English: We left early.
J'ai mangé des pommes.
English: I ate apples.
Les lettres que j'ai écrites sont sur la table.
English: The letters that I wrote are on the table.
Ils sont arrivés en retard.
English: They arrived late.