A man is an adult male human. When a boy grows up, he becomes a man. The word contrasts with woman (an adult female human) and with boy (a male child).

The word also means human beings in general, without reference to gender. When someone says “man has walked on the moon” or “the rights of man,” they're talking about people as a whole. This usage feels old-fashioned to many people today, who often prefer words like humanity, humankind, or people to make it clear they mean everyone.

You'll also hear man used as an exclamation of surprise or emphasis: “Man, that was an amazing catch!” In this case, it's just an expression and doesn't refer to anyone's gender at all.