Plot resetting is when you know a villager is moving into your town, so before you launch the game with your mayor character, you create a new character and run around your town to see where the plot was placed.
By making a new character you can shut the game off without saving to reset where the plot is, so you can ultimately get their house in a spot you like.
But this only works if the villager is placing the plot down that day (roped off plot not actual house) and only if you launch a new character before the mayor. Launching the mayor will permanently save where there villager is, so you can't shut off without saving and reset their plot location.