What is the perfect tense

Bonjour Lyn, There isn't one perfect tense - "perfect" describes several tenses: present perfect, past perfect, future perfect, etc. Grammatically, it means "complete" in contrast to the imperfect which is incomplete.

The past perfect is sometimes referred to as the perfect tense, I think in contrast to another past tense known as imparfait, imperfect. The past perfect is also known as le passé composé.