A naval mine is an underwater, or at least partially under water, military ordinance that does not require any maintenance and can be left in one spot for years. The goal of these devices was to sink enemy shipping.
They were widely used by both sides during the war. The first ship sunk was the USS Cairo in 1863, in the Yazoo River.
Once set, they would simply float and wait. The early mines would often not detonate and would often be duds. As both sides learned how to create and use them more effectively the instances of a mine failing became less frequent.
The Confederate navy made good use of mines and sunk many Union ships that were unlikely enough to run into one.
The Confederates would go on to sink, or severely damage, a great number of ships in the Union navy. The most famous use of a mine during the Civil War was when the submarine CSS Hunley stuck a mine attached to a lance into the hull of the USS Housatonic sinking the great Union warship.