Plot resetting can refer to a few different things. The general method is that, when you turn on your game for the first time of a new AC day (after 6:00 AM), instead of loading up your mayor, create a new character and go through Rover's spiel. This is important because there's a difference between making a new character and loading your mayor. When you load your mayor, your town is simultaneously generated and saved. This happens when Isabelle is doing her spiel. However, when you make a new character, the town is only "generated." It doesn't save until you get your TPC.
Because it isn't saved, and because the town has variations for each given day on how it is generated, you can do a few different things. For starters, if you have less than 9 villagers, you have a chance of getting a random new villager. If you do the reset trick, you can try and pick what villager you get. Also, if you know that you have a villager moving in, you can change where they put their plot. In both these cases, as long as you don't save, every time you make a new character, the plot changes. You can also do campsite resetting since, again, every time you make a new character, you have a chance of getting a different camper. In all these cases, once you find something you like, you go until you get your TPC, save the game, then load up your mayor and you'll have whatever you reset for. Sorry this was so long-winded! If you have more questions about this, ask away!
