Plot resetting is where you reset the fenced-off villager plots that appear the day before a house appears. Like, you know those fenced-off plots of area that have a little sign that says something like "Reserved for <villager's name>"? Those are villager plots.
What people do is they make a new character BEFORE LOADING AN EXISTING CHARACTER ON THAT DAY, and they wander around the town looking for a plot. If it's in an inconvenient place or not the villager someone wanted, they turn off the game without going to isabelle, getting a house, etc etc, thus not saving the game.
Then they reload the game and try again until they are satisfied with the placement/villager, and they go through the thing with isabelle, talking to tom nook and getting a house, and then back to isabelle to get the TPC, so that they can save the game. After that, the game is saved along with the plot placement and villager, and they can delete the character they made for plot-resetting if they want.
It's more simple than it probably sounds! I do it all the time!
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Also, if no plots appear after 3 or 4 resets, a villager plot is most likely not going to appear that day. Thus, you can probably quit the game and go into your Mayor or whatever,
BUT I highly recommend you save the game anyway to be safe. Hope I helped! :3