This is one of those saying that has had its meaning twisted up. "Reap what you sow" turned into "You reap what you sow" A small change in wording, but in meaning a large one. "You reap what you sow" tends to be used to imply the inevitability of consequence. Your actions in the past lead you to a future you can't escape. The consequences of those actions.
On the other hand "Reap what you sow" rather than being a foreboding warning of consequence is advice. To sow is to plant. If you plant a crop it's generally good practice to reap it (harvest). Otherwise all your previous hard work of sowing goes to waste.