Elle est allée au marché.
English: She went to the market.
The 'accord du participe passé' in French means that the past participle sometimes changes its ending to agree with the gender and number of the noun it refers to.
You use the past participle agreement in French mainly with verbs that use 'être' (to be) as the auxiliary verb, and sometimes with 'avoir' (to have) if the direct object comes before the verb. The participle agrees in gender and number with the subject (with 'être') or with the preceding direct object (with 'avoir').
Elle est allée au marché.
English: She went to the market.
Ils sont partis tôt.
English: They left early.
Les pommes que j'ai mangées étaient délicieuses.
English: The apples that I ate were delicious.
Nous avons vu les filles.
English: We saw the girls.
La lettre que tu as écrite est sur la table.
English: The letter you wrote is on the table.