eh, that's when somebody is getting their villagers to continuously move out, getting in new replacement villagers, getting those to then move out, etc. in a (longterm?) cycle of flowing them out...
usually its because they lost a particular villager and want to cycle through all the stock of villagers until they get back the one they lost; or, they are trying to have every villager at least once; or, they are just cycling through them all so they can sell the "popular" ones on this or other forums to other players....
I also wasn't entirely sure what this meant until recently, the people above explained it incredibly well but I am still unclear on how you can get rid of villagers to cycle through them so fast. If anyone could please explain this that would be incredible, thanks.