It doesn't really "mean" anything. It is not exactly translatable.
It EXPRESSES various types of EMOTIONS in reaction to something perceived.
Sometimes the emotion is disappointment or exasperation.
EX: "You did really well on the exam but others did better" - "Awhhh! Man!"
EX: "Your car was just hit by a bus" - "Awwh! Man!"
Sometimes "Awwwww" (with varying numbers of "w"s) expresses a different emotion, such as "That is so sweet/cute".
EX: "You see a tiny kitten playing with a puppy" - "Awwwwww!"