Dutch Roll is a combined roll : yaw oscillation. It is driven by the lateral and directional stabilities (which respectively are rolling moment due to sideslip and yawing moment due to sideslip). The DR oscillation occurs when the damping in either yaw or roll is very poor - this can be for various reasons, but a large side area forward of the CG is probably the single most common reason.
If it occurs it is driven by turbulence, or by rapid rolling manoeuvres. It's seen by the nose describing a horizontal figure of 8, or the wingtip describing an oval.
If it occurs, it's generally fixed by improving damping in the axis which has the largest magnitude of stability - that is, if the horizontal side of the oval described by the wingtip is longer then directional damping needs improving - possibly by a damper, possibly by a bigger fin. If the vertical side of the oval is longer, improved roll damping is needed.
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