There were few professions in that era which earned a man the kind of money Gatsby made. In addition, Gatsby was receiving calls from all hours of the day and we as a reading audience are never truly told what he does. We just knew he received calls from Detroit or Chicago. These are places one (or another character like Tom) could justifiably believe he was communicating with for the transportation or distribution of alcohol.
Tom's charge was well-founded. Pohnpei's above description of Tom's motivation for such charges is correct.
Tom says that Gatsby was a bootlegger -- someone who sold illegal alcohol during Prohibition. He also accuses Gatsby of having been a racketeer and a swindler. This happens in Chapter 7.
The reason that he says this is that he does not want his wife, Daisy, to go off with Gatsby. He is trying to show Daisy that Gatsby is really just a criminal. If this is true, then Daisy will, presumably, not want to go off with him and she will stay with Tom instead.
So Tom is saying bad things about Gatsby to try to discredit him in Daisy's eyes.