The impersonal pronoun ‘man’ is the German equivalent of ‘one’ (or ‘they’) in English. As the name suggests, it is used to denote a non-specific, generic individual that functions as a placeholder and is associated with the passive voice as an alternative form. Look at the following sentence in English and German:
One cannot do that! Man kann das nicht machen!
In English, the passive voice and impersonal pronouns are often encouraged in formal writing, as is the case in German. In fact, you will encounter it frequently as you learn German. Here are a few more examples of the use of the impersonal pronoun in other cases (‘man’ = nominative):
Akkusativ Dieser Lärm (noise) ärgert einen! / This noise is annoying!
Dativ Die kalte Luft macht einem etwas aus. / The cold air is bothersome.
The English equivalents of the above examples are clearly not word-for-word translations because it is expressed using a different structure in English — the impersonal pronoun ‘one’ is only used in the nominative case (‘it’ is used in nominative and accusative cases). Further, the use of the possessive adjective ‘sein’ is required for the impersonal pronoun ‘man’ (in English, this is “one’s”):
Man sollte auf sein Auto aufpassen.