A 12-string acoustic guitar is a guitar with 12 strings in six doubled courses. It’s played the same way a standard, six-string acoustic guitar is played, but it is known for having a fuller, warmer sound.
12-string acoustic guitars require unique strings because some of the strings are doubled in octaves. For example, the D'Addario XT Phosphor Bronze Acoustic Guitar Strings are sold with the bottom four strings doubled in octaves. This is what gives the 12-string acoustic guitar such a rich sound.