We are given some clues that Meyer Wolfshiem is involved in the Mafia. The most notable detail is Nick's observation and question about his unusual cuff links. Wolfshiem tells him that they are human molars. Frankly, that is a disgusting thought -- so it begs the question, "What kind of person would wear human cuff links and why would that person wear them?" The answer is someone who is a ruthless and wants to send that message by a public display of the trophy of human teeth. Earlier in this chapter it is suggested that Gatsby is a bootlegger, so our meeting of a character like Wolfsheim a few pages later rounds out that picture. In addition to the cuff links there is also Wolfshiem's comments about being friends with Rosy Rosenthal who was murdered outside a restaurant. This sounds like a mob hit. After he leaves the table it is revealed that Wolfshiem is the man who fixed the World Series back in 1919. That cheating scandal is still a black spot on professional baseball today and was obviously a huge controversy back then. When we consider the totality of the details, we are left to assume that he is an important man of the mafia and that leaves us wondering what to think of Gatsby if he has business connections with a man like that.
If I remember correctly, it implies that Gatsby has earned his money in a not so legal manner, Wolfshiem is a shady character and Gatsby's mysterious and secret past leads Nick to think that Gatsby has a lot to hide.