They are a physical phenomenon where, when the device goes above a certain speed, they become more and more susceptible to the board violently oscillating back and forth, uncontrollably, oftentimes resulting in severe crashes where the rider is thrown from the board, possibly leading to significant injury as well as damage to the board.

Pretty much every experienced rider has experienced then at some point.
They can be reduced to an extent by hardware modifications or changing the way one rides, but, because they arise out of a physical phenomenon inherent in the mechanics of the boards, they can not be eliminated entirely.
Additionally, riders can learn to make adjustments to how they ride to respond to them when they occur.

A lot has been written by many people online about speed, wobbles age how to prevent them, unfortunately, almost all of it wrong. Speed wobbles arise out of a physical phenomenon called the Hopf bifurcation. In systems affected by the Hopf bifurcation, the physical parameters of the system create a specific threshold, below, which, the system remains stable in a static equilibrium. But above that threshold, they transition to a state where they enter a dynamic equilibrium, where, instead of remaining in a stable state, they oscillate back-and-forth between two extremes.

This effects numerous other physical systems, which have their own Hopf bifurcation points where they transition from stable to oscillating states, from skateboards to EUCS, to shopping cart wheels, to population dynamics in predator/prey relationships in biological ecology
The way this affects skateboards, is that when they are riding slowly, the static, stable equilibrium state is one in which the rider is riding straight forward with the wheels parallel to the deck, and the trucks perpendicular to the street. But above the Hopf bifurcation point, the board transitions to a state where it is no longer stable in static equilibrium, riding forward with the wheels in a straight line, but rather, is now becomes stable in an oscillating equilibrium, where the wheels turn to the left, and then turn to the right, and back again, and the trucks alternately angle towards the deck on the left and then back toward the deck on the right, causing the board to steer quickly left and right then left again back-and-forth, violently shaking the rider until he or she can either regain control of the board, or is thrown off entirely, at high speed.

This is made even worse by inexperienced riders because their autonomic nervous system, which is attempting to keep their body upright, responds to this violent oscillation in exactly the wrong way, responding to the boards veering to the left by instinctively attempting to turn back to the right, but doing so at exactly the wrong time after the board has already transitioned to turning to the right on his own. As a result, the rider’s actions only serve to amplify the turning of the board to the right. When the rider senses that the board is turning too quickly to the right, they again instinctively attempt to correct it by veering to the left, but again correct just as the board is already steering to the left on its own, thus, the rider amplifies the oscillations that began because of the Hopf bifurcation in exactly the way that makes these oscillations become amplified. And the faster the rider goes, the more violent the oscillations become, the harder they become to control, and the more likely the rider is to be thrown off the board.

Experienced riders are able to regain control (or never lose it in the first place) through a number of techniques, but also, after significant riding experience, can avoid them altogether, riding through and beyond the Hopf bifurcation point, where they basically train, retrain, or untrain their nervous system so it stops overreacting to the oscillations under their feet, and instead merely let’s the board oscillate under their feet and “rides them out“.

Ultimately, you don’t counter speed wobbles by modifying your board hardware, you reduce or eliminate speed wobbles by training your brain through practice on how to ride fast.