The cotton gin is a device for removing the seeds from cotton fiber. Such machines have been around for centuries. Eli Whitney's machine of 1794, however, was the first to clean short-staple cotton, and a single device could produce up to fifty pounds of cleaned cotton in a day. This made cotton a profitable crop for the first time.
After this invention, the yield of raw cotton doubled each decade after 1800. Demand was fueled by other inventions of the Industrial Revolution, such as the machines to spin and weave it and the steamboat to transport it. By mid-century America was growing three-quarters of the world's supply of cotton, most of it shipped to England or New England where it was manufactured into cloth. During this time tobacco fell in value, rice exports at best stayed steady, and sugar began to thrive, but only in Louisiana. At mid-century the South provided three-fifths of America's exports—most of it in cotton.
See the Teaching With Documents Lesson Plan: Eli Whitney's Patent for the Cotton Gin at the National Archives' Digital Classroom.