means The act of cutting and gathering a crop, or by extension, receiving the consequences—good or bad—of your earlier actions.

from From Old English 'reopan' or 'ripan,' to cut grain, related to 'ripe'—the grain ready for harvest. The agricultural sense came first; the moral 'reap what you sow' meaning was sharpened by its appearance in the King James Bible, which planted the phrase deep in English. The grim Reaper, scythe in hand, is the same image turned toward a darker harvest.