In his Democratic National Convention speech on 27 July, Barack Obama said:
Let me tell you, eight years ago, you may remember Hillary and I were rivals for the Democratic nomination. We battled for a year and a half. Let me tell you, it was tough, because Hillary was tough. I was worn out. She was doing everything I was doing, but just like Ginger Rogers, it was backwards in heels. And every time I thought I might have the race won, Hillary just came back stronger.
What is it meant by "it was backwards in heels"?