A wheel hub is a round component with studs (which are similar to the threaded end of a bolt) in it that are used to mount a rim of a wheel to the vehicle. This part also often houses the wheel bearing, or is connected to it in some way.
Wheel Hub Spacers
Wheel hub spacers are a quick and easy (but not always the best) way to widen the wheelbase of nearly any vehicle. If only the spacer is applied, the other suspension components might not be aligned properly, and this could make the vehicle behave unpredictably.
Why Wheel Spacers Are Dangerous
Wheel hub spacers don't pose a thread on their own, but when installed with wheel studs of insufficient length, there may not be enough threads for the lug nut to tighten down on, and this increases the odds of the lug nuts coming off and/or breaking the studs. This can result in a wheel coming off at high speed, creating two points of danger: a vehicle that is unlikely to be controlled, and a wheel that has been launched somewhere.
Even if there is enough thread, the amount of thread being grabbed on to has changed, and the appropriate lug nut torque will have changed, too.
This doesn't mean that any vehicle you see with wheels spread apart further than stock is dangerous. There are multiple ways to use spacers in a safe way, and sometimes this effect is achieved without any wheel spacers. With spacers, the simplest way to keep things safe is to install longer (and sometimes stronger) wheel studs. A wider stance is also able to be achieved with wheels that have a different offset, or with different suspension components.