The Cotton Gin was the most significant change in the 1800's because it increased cotton production dramatically, cotton became a cash crop, and the need for slaves became greater.
Because of the invention of the Cotton Gin, cotton production increased dramatically. Where before one slave could only pick about a pound a day, the cotton gin could do that, and more in less than an hour. "With Eli Whitney's invention, cotton lint could be produced quickly and efficiently at up to 50 pounds per day." (reference.com)
How the Cotton Gin works: "First, the cotton balls were put into the top of the machine. Next, you turn the handle, which turns the cotton through the wire teeth that combs out the seeds. Then the cotton is pulled out of the wire teeth and out of the cotton gin." (score.rims.k12.ca.us)
Originally, the only 2 cash crops that America had were Indigo and Tobacco. After the Cotton Gin was invented however, cotton became a cash crop as well since southern farmers could make a living out of it.
You would think that because of the Cotton Gin, less slaves would be needed since it required less people to use the simple machine. On the contrary, the need for slaves became greater, since more people were needed to pick more cotton. Slaves also became more expensive. "More people were needed to harvest all the cotton. Slaves were used for this this task. The need for slaves increased and caused more disagreements between North and South. Because the need for slaves increased, the value of the slaves also increased." (score.rims.k12.ca.us)
The way it affected people directly is that it increased slavery and southerners received more money from the amount of cotton that was produced.