It means in some states, such as California and Oregon, but likely others as well, workers comp is the sole relief for an injured worker. All injuries are considered no-fault and the injured worker has no right to go after their employer unless they can prove that their employer intended to do harm. The injured workers only gets the medical treatment and whatever disability payout workers comp tosses their way.

So, if your supervisor is drinking at work and ignoring warnings about dangerous equipment and then you get hurt by that equipment, you cannot sue. If your supervisor wrote an email about how they didn’t fix some equipment so that it would hurt someone and you get hurt by that equipment, you might be able to sue.