To thresh means to separate grain from the rest of the plant after harvest. When farmers thresh wheat, they beat or crush the stalks so the edible seeds fall away from the chaff (the dry, papery husks that aren't good to eat).

For thousands of years, people threshed grain by hand, hitting bundles of wheat or rice with sticks called flails, or by having animals walk over the stalks to break them apart. It was exhausting work that took days or weeks. The invention of the threshing machine in the 1700s changed farming forever, doing in hours what once took weeks.

You might also see the word used more broadly to mean beating or thrashing something violently. A swimmer might thresh through rough ocean waves, arms and legs working hard against the water. A fish caught on a line might thresh about, struggling to break free.