It can be used to refer to:
* A mail carrier: "Le courrier est arrivé." (The mail carrier has arrived.)
* Mail: "J'ai reçu du courrier aujourd'hui." (I received mail today.)
* A messenger: "Il est arrivé en courrier express." (He arrived by express courier.)
Keep in mind that the gender of a noun is important in French grammar, affecting the agreement of adjectives, articles, and pronouns.