der Mann – man (Pl. form – die Männer – men)
So you see ‚der Mann‘ is noun (‘Substantiv’) whereas
‘Man’ is pronoun which means ‘you’ or ‘one’.
Also note that as ‘Mann’ is a noun so ‘M’ is always capital but in case of ‘Man’ it depends on its position in a sentence (here ‘M’ can be capital or small)
Let’s see some examples:
Ist er der Mann meines Lebens? - Is he the man of my life?
Der Mann, der in der Bibliothek arbeitet, ist mein Bruder - The man who works in the library is my brother.
Alle Männer Müssen Kämpfen - All Men Must Fight (Look ‚Männer‘ is the plural form of Mann)
Now,
Man kann das sagen – you can say that or one can say that.
Darf man hier rauchen? – Is it allowed to smoke here? But more literally it means Can someone smoke here? Is it allowed?