Brake checking is when a driver abruptly and intentionally applies their brakes, often without any reason, with the intent of causing the following vehicle to collide with them, or more likely, put them in fear of colliding with them. Brake checking is both illegal and stupid and can lead to rear-end collisions, property damage, injuries, and even fatalities.
The other rise with brake checking is that it is arguably intentional in most cases, not negligent. So your car insurance can disclaim coverage because it is an intentional act, leaving the brake checker on the hook for all of the property damages and injuries in the crash.