Answer and Explanation:
NOTE: I will use the page number from the book I own. Change it if it does not match the page in your book.
"The Great Gatsby" is a novel by F. Scott Fitzgerald whose themes concern the degradation of the American dream and society. In chapter 4, the narrator, Nick, is riding in Gatsby's car with Gatsby himself. Up to this point in the story, very little is known about Gatsby. He is this mysterious millionaire who seems to have taken a sudden interest in Nick.
While driving - and speeding -, Gatsby is tagged along by a policeman. He shows the policeman a white card that immediately releases him from having any kind of trouble. Nick asks him what it was, to which he gets the following answer:
‘I was able to do the commissioner a favor once, and he sends me a Christmas card every year.' (page 74)
That interaction already reveals a lot about Gatsby. We know he is somehow influential. The Christmas card he showed the policeman is enough to keep him from getting a speed ticket, and probably would keep him from having any trouble with the police whatsoever. Even though we do not know what the favor was, Gatsby is obviously relying on it to get away with wrongdoings, which leads us to think it was likely something shady.