On this page of the guide to Back 4 Blood you will read about Trauma, which is a new mechanic of the game. You will learn what difficulties come with the Trauma, how how you can get it and how to remove it.
Trauma is a game mechanic that causes a certain part of the red health bar to turn red-black. This means that the health bar, which is red-black, is impossible to cure. That's what trauma is. It lowers the character's maximum health.
You will get the Trauma if you keep taking damage when injured. Depending on the difficulty level, the Trauma may fill up more slowly or quickly.
To reduce the Trauma, you can have a Doc on your team. In order to completely remove it, you need to go to a first aid station. They are often found in buildings and safe rooms. Approach them and heal yourself. Pay attention to their description, though, because some of them are free, while others are paid (the cost is 400 CR).