Dutch Roll is an old sailing term that described how Dutch warships moved. Because of the shallow draft needed for their home waters, the ships had small shallow keels, and thus little roll and yaw stability. They would roll in one way and yaw in the opposite way, then reverse the motion.
Similarly, an aircraft with too little yaw stability and too little roll stability can enter a coupled motion where a yaw movement causes a roll in the opposite direction. If the yaw oscillates, than the roll does as well.
For helicopters, the reason is usually a high tail rotor and weak roll stability. If the aircraft hits a small gust, the high tail rotor causes the restoring yaw moment to be coupled with a strong opposite roll moment (the high tail rotor makes the yaw come back home, but rolls the aircraft in the opposite way.)
As several posters have said, the best way to handle a dutch roll is to apply small pedel motions to stop the yaw. This keeps the roll in check as well. If a yaw SAS is available, it will magically make the roll behave.