This is a generic Physics concept:
A system with oscillatory stability is an oscillatory system whose interaction with the outer world tends to decrease the oscillation amplitude (or energy), and for a system with oscillatory instability the interaction with the outer world tends to increase the oscillation amplitude.
I think that PhilM refer to the "Deutch Roll".
The wings diedral increases the lateral stability of a plane, but it creates an oscillatory system on the rolling axis.
When the diedral angle is higher than some threshold, the energy given to this system by the sideslip air stream
is sufficient for making the oscillator instable, and thus the plane tends to have increasing rolling oscillations.
The rudder fin surface increases the directional stability of the plane and has a stabilizing effect on the rolling oscillator
by reducing the lift of the wing on the slip side and increasing the lift of the other wing.
Luc