muddyboots wrote:what is it and why is it good to improve it?

First reason is that with lateral (yaw) instability you have to apply rudde rforces to counteract it.
The application of rudder forces induces significant lateral loads into the aft fuselage.
In a dive (say a 51 at .8M) this is NOT a good thing as the dynamic pressure forces on the airframe are high and the aircraft is probably close to limit/ulmate loads.