It is a particular fly. EX: Wet Flies, Dries, Woolly Buggers, etc.
Everyone has their favorite, so by throwing a swap, you get a bunch of different flies to try. If 12 people join, you tie 11 of your ONE favorite pattern (all the same) and send them in with a self addressed stamped envelope of postage money for a return. The other 11 will do the same. By tying 11 in a group of 12, just means you don't get yours back.
Really is fun and like Christmas, specially when you get in swaps where everyone is very talented.