It is a place/event where Pokemon compete in races and similar events.
This feature was introduced in Heart Gold/Soul Silver.
>Pokéathlon is a type of competition involving Pokémon racing and similar athletic competitions, in the manner of Olympic track and field competitions. It was introduced in Pokémon HeartGold and SoulSilver.
The Pokeathlon Dome, where the competitions are held, is north of Goldenrod City, next to the National Park.
>The Pokéathlon is handled similarly to Pokémon Contests and Super Contests in the Hoenn and Sinnoh regions, as a sidequest to complete while the player challenges the Gyms and the Pokémon League.
Just like in contests, four people compete at once.
Here is a list of all events held at the Pokeathlon.
Some notable contestants that appear in the events are:
- Falkner
- Maylene
- Jasmine
- Kuni(a kimono girl)
- Whitney
Source
Hope I helped.
