Pokémon swarms are things that can happen in the games after getting the National Pokédex from generation 2 onwards. They allow the player to catch Pokémon that would normally be rare or unobtainable.
How to tell when a swarm happens in Black/White:
The gates when you go to enter a route will announce that there is a Pokémon outbreak on a certain route. The current Pokémon swarm will change every day at midnight, though you can manipulate it by Soft Resetting the game if you haven't saved on that day. When the game has been restarted, a different Pokémon will appear.
The route will look the same as usual, so the only way to tell is by reading the message on the Route gates. All you need to do is run about in the grass and you should eventually find the rare Pokémon.
Hope I helped!
Sources: This video, Bulbapedia and Serebii
