Elle est allée au marché.
English: She went to the market.
The 'accord du participe passé avec avoir et être' is a French grammar rule about how the past participle changes (agrees) in gender and number, depending on whether the auxiliary verb is 'avoir' or 'être'.
You use this rule when forming the passé composé and other compound tenses in French. The agreement of the past participle depends on which auxiliary verb is used and the position of the direct object.
Elle est allée au marché.
English: She went to the market.
Ils ont mangé les pommes que j'ai achetées.
English: They ate the apples that I bought.
Nous sommes arrivés en retard.
English: We arrived late.
La lettre que tu as écrite est sur la table.
English: The letter that you wrote is on the table.
Elles sont parties tôt.
English: They (feminine) left early.