Dutch roll is yaw coupling into roll, causing a motion that is a combination of roll and yaw, and is found in forward flight.
The name supposedly comes because is seems like the motion of a speed skater, and at the time it was coined, most of the speed skaters were Dutch (at least that's the reason that appears most often)
A sideslip typically starts the motion, due to a gust of wind upsetting the torque balance between the main rotor and fuselage. The helicopter yaws, and this produces a change in the sideslip. The sideslip produces a side wind on the fuselage and main rotor, which generates some rolling. Most of the time, the motion damps down immediately, and all you would see or feel is a slight yaw and perhaps a small change in roll attitude.
In swept wing airplanes, the motion can continue, and for this reason yaw dampers are fitted to stop any yawing motion.
Some helicopters are more affected by it than others, and changes in CG position (both longitudinal and vertical CG) can change the effect of a small change in sideslip.
It's not very prevalent in helicopters - I just had the pleasure of sitting in the back of a Bell 407 for several hours yesterday and the mild turbulence caused noticeable yawing motion, but almost not perceivable roll.
And any helicopter with an attitude hold in roll will also mask the problem.