to reap, to harvest: to cut down standing grain at harvest time.
This is the ordinary farming word for bringing in the grain crop. A worker takes the standing stalks of barley or wheat and cuts them, so the word covers the whole picture of harvest: the field, the cutters moving through it, and the gathered grain. The people who do this work are simply called reapers, and many of the word's appearances are scenes of them at work, most memorably in the book of Ruth.
Because harvest was the payoff of a year's labor, the word became a natural image for consequences. The Bible turns it into a proverb again and again: you reap what you sow. A person who plants trouble harvests trouble, and one who sows kindness gathers a kind return. The prophets use the same picture for judgment, where a nation that has sown wrongdoing now brings in the bitter crop.
The law also attaches a famous rule to this word. When Israelites reaped their fields, they were not to cut all the way to the edges or go back over the missed stalks, but to leave that grain standing for the poor and the foreigner to gather. So the act of reaping carries, in the law, a built in reminder to leave something behind for those in need.