Scatter is often reffered to as the slot players best friend. A scatter is a wild card that can show up on any reel to unlock a bonus, extra spin or some other exciting bonus fuction. Since the scatter works different on different slot machines, it is recommended that you always read up on what applies to the machine you are playing to make use of the scatter best way possible.
All modern slot machines offers some kind of scatter. Sometimes it is necessary to collect three different scatter on the same spin to trigger a bonus round. Usually two scatters are enough to trigger a bonus opportunity.