you do a great thing ! include the doing ,acting ,talking and so on

means awesome ! cool !

牛 in english is equivalent to adjectives like fantastic,marvelous,spectacular,incredible,amazing...etc.We usually use it in oral chinese.Although 牛 is an informal word but it is utterly a very high compliment.

actually this is not "traditional" chinese,it's just some kind of trenky,people created it recently and you'll never see it in formal books as textbook or poems

meaning you do a good job