Popular sovereignty, in the context of US history, is used to refer to a system in which each state would be able to decide for itself whether it would be slave or free. Popular sovereignty was a change from the previous system in which Congress had decided which areas would be able to have slaves and which would not. Congress had done this, for example, in the Missouri Compromise.
Popular sovereignty was significant because it helped lead to the Civil War. It helped to cause conflict in the various territories as people fought over whether they would have slaves. The worst of these conflicts was in “Bleeding Kansas” where pro- and anti-slavery forces took turns inflicting atrocities on one another. This conflict helped bring about the Civil War.
Popular Sovereignty was a political doctrine that the United States subscribed during the mid-nineteenth century, which stated that the settlers of a given territory would have the sole right to decide whether or not slavery would be permitted there. The First proponent of this principle was Senator Lewis, who put the idea forward opposing the Wilmot Proviso. But, Senator Stephen A. Douglas was the leading proponent of popular sovereignty in 1854 because he popularized it. The principle Popular Sovereignty was conjured in the Compromise of 1850 and later in the Kansas Nebraska Act that took place in 1854. The events in “Bleeding Kansas” exposed the weakness of the doctrine.