This is a pun on the word "downhill". When someone goes to the Olympics, it's probably the best thing that ever happened to them. They are at the highest point in their life, so anything after that decreases in greatness. A common phrase people use to describe this phenomenon is that it's "all downhill from there". Downhill is also the direction you go when you ski and the joke is about a skier.
A skier who makes it to the olympics finds that it is all downhill from there .