"man" is an impersonal pronoun that substitutes a noun. It is used to signify a person or several people, but no one in particular. So it is often used to express generalizations.

Example:
Hier darf man nicht Fahrrad fahren. (= That applies to everyone.)

"man" is always used in the nominative and the singular, even when it applies to "everyone". There is no plural form of "man".