An event is basically where Nintendo officially releases a promotion where you can get special Pokémon and items and something like that. The event celebi is one of their promotions, and the promotions usually last for a little while. Most of the Pokémon are rare, or have special moves or items and my have a special use. If you relocate the celebi to your black and white game, you can get a zorua in castelia.
Hope this helps!