The general rule is that workers' compensation benefits are an injured employee's sole and exclusive remedy when he or she is injured by an anticipated and foreseeable risk of his or her employment. The injured employee can't sue their employer in an actual court of law for an injury that was sustained in an accident at work. An exception to this rule arises when that employee is injured as a result of the negligence of somebody who wasn't a co-worker.
Put the general rule and exception in the context of a Caltrans employee who was performing maintenance on a guardrail on a state highway. He's working 15 feet in front of a Caltrans truck when a tractor-trailer clips the truck. The force of impact propels the Caltrans truck forward, and it strikes the Caltrans employee who was working on the guardrail.