The Kansas-Nebraska Act passed May 30, 1854 allowed the territories of Kansas & Nebraska to decide themselves wether or not they would adopt slavery. This infuriated the Northerns who followed the Missouri Compromise and the Southerns pushed for slavery in the territories. Due to this two legislation in Kansas many people opposed each other giving Kansas the nickname "Bleeding Kansas" President Pierce had to send in Federal Troops to cease the civil unrest.
Too summarize the Kansas-Nebraska act stated that the territories could vote wether or not they should adopt slavery.