the void is where villagers go when no one wants them - so when they move out, instead of going to someone elses town, they'll go to the "void", which is essentially a blank space where a villager goes temporarily after they move out, providing they dont go to someone elses town.
clearing the void is basically getting rid of the villager in the "void" so they don't move into someones town who doesn't want them. to clear the void you simply go to someones town who has a full 10 villagers and the "voided" villager will be removed.