It is not for absolutely sure that she did cheat. She was accused of cheating but then the people who accused her sort of took it back.
Anyway, what she was accused of doing was moving her ball from a bad "lie" during a golf tournament. In golf, wherever your ball lands, you're supposed to hit it again from there. If you move it out of a place where it's hard to hit (a bad lie), you are cheating. That's what Jordan was accused of.
You can find this answer not too far from the end of Chapter 3.
Jordan Baker in The Great Gatsby is accused of and allegedly moved her golf ball during a tournament, in order to gain a better position for her next shot. This is cheating in golf.
This reflects on Jordan's character. Something like this just isn't a big deal to her. She is extremely careless when driving as well. She assumes other drivers will be careful and will get out of her way.
The accusation of cheating and her driving suggest Jordan feels a sense of entitlement. She deserves to win, whether she earns it or not; and it is up to others to get out of her way.
These values clash with Nick's Midwestern values, or at least that's how he presents the situation to the reader. And he ends up dating Jordan and enjoys being with her, so he probably doesn't have any reason to make Jordan look worse than she is.