What is le dernier mois mean?

Answer 1

Le dernier mois means 'the last month (of a series)' in French.

Décembre est le dernier mois de l'année > December is the year's last month.

Au départ tout allait bien, mais le dernier mois a été difficile > at the beginning all was fine, but the last month was more difficult.

BUT if you want to speak of something that took place last month, the resembling expression 'le mois dernier' is better: she had a baby last month > elle a eu un bébé le mois dernier. She visited her grandparents last month > elle a rendu visite à ses grand-parents le mois dernier.