A sound system is a group of sound engineers, DJs and performers/MCs who work together to put on a live show. Sound systems are most closely associated with dancehall music, and within that context, DJs are often called "selectors" and MCs are often called "DJs." Sound systems have a particular brand and identity usually, not unlike a band or other group of musicians. In dancehall music, sound systems often have "sound clashes" or competitions with each other.
Although modern sound systems are associated with dancehall music, sound systems date back to 1950s Jamaica. Sound systems have often played a central role in Jamaican politics.
For more information on sound systems, see this profile of Louise Frazer-Bennet.