When Tom and Daisy attend one of Gatsby's parties, Gatsby makes a point of introducing Tom as "the polo player."
He took them ceremoniously from group to group:
"Mrs. Buchanan . . . and Mr. Buchanan--" After an instant's hesitation he added: "the polo player."
"Oh no," objected Tom quickly, "not me."
But evidently the sound of it pleased Gatsby, for Tom remained "the polo player" for the rest of the evening.
(Fitzgerald, The Great Gatsby, mrbye.com)
The party has many famous or influential guests, including actors and other celebrities, and in the vein of his own constructed past, Gatsby attaches a purpose to Tom's name instead of just introducing him as a fellow wealthy individual. The addendum makes Tom seem more important than he actually is, and also shows Gatsby's disdain for the shallow pursuits of the wealthy; Gatsby made his own money through hard work, while the others mainly inherited theirs and spend their days in leisure. The description is intended both as an oblique insult and as a way for Gatsby to have fun at Tom's expense; Tom doesn't quite pick up on it, as he is just happy to be seen at the party by all the famous people.