A plot is a piece of land being used (or having just been used) for the cultivation of crops. Each one is 4x4 squares. One is created when the farmer plows grass or snow, and then proceeds through the life cycle of the crop as follows:
If a plot has been fertilized, it will appear to sparkle as if dusted with magic sparkly dust until maturation percentage 100 (maturity), and its crop will appear larger and more bountiful.
In the context of the game, the size of a farm is measured in plots, not in squares, and there are always two extra squares on each edge, so the current maximum sized farm is 24x24 plots, or (24*4+2)x(24*4+2) squares, or (96+2)x(96+2) squares, or 98x98 squares.