French Anterior Past
Le Passé Antérieur is a literary past tense of the indicative mood. It expresses an action that was completed immediately before another past action.
It is rarely used nowadays, but it can be found in formal writing or literature. It is useful to be able to recognise Le Passé Antérieur when reading classic French literature or poetry for example.
The English equivalent would be the past perfect, which is formed with "had" + past participle, while, in French, Le Passé Antérieur is formed with avoir or être in Le Passé Simple + past participle.
For example:
Après qu'elle eut lu la lettre, elle partit sans dire un mot.After she had read the letter, she left without a word.