Plotting and farming are part of the same thing. You have to plot first, plotting is basically creating a giant bingo card that can be taken from one game to the next (aka the block challenges). You can stop plotting after the first one, or continue until you can't afford more drives. It is your choice, the more bingo cards you have, the higher chance you have of winning. Farming is the long typical "mining" part. You PC will receive a new blockchain challenge and scan your bingo cards for an answer. If there is no BINGO, it will check again on the next plot. Reading from the farming drives uses very little system resources, you can play games, work, etc during this process. Plotting on the other hand depending on your settings can peg your CPU and RAM at 100%. But again, you only do this if you want more bingo cards.