Swarming refers to a daily event where one of something like 20 different pokemon have a chance of appearing. You can find out which one is swarming that day by going to Sandgem Town and talking to the Professor's assistant's (Dawn or Brandon or whoever it is supposed to be) sister in their house. She talks about how she saw a news report about how a certain Pokemon is swarming and what location (usually a route) they are swarming at.