In Warhammer 40k: Rogue Trader, you will buildup wounds and injuries as you take damage. Thus, your character will always get an injury whenever they take damage from something, like a trap. They will get an injury if they lose more than half of their health in one attack. These are a few various ways to get injuries in W40k: Rogue Trader.
So, three injuries will get you a trauma. While injuries don't often mean a lot, trauma lowers your character's stats, therefore you shouldn't usually have trauma on your character. Your character will experience trauma if he loses all of his health and passes out, or if he suffer three injuries, which will add up to become a trauma. This means that you will lose the three injuries you have but gain a trauma instead.
So how do we stop injuries and trauma? Well, that's where the medi kits come in—a way to stop all of that from happening. Now, there are two types of injuries: a fresh injury is one that treated with medi kit, and a fresh injury becomes an old injury if it is not treated with a medi kit within three rounds. Old injuries are treated similarly to fresh injuries, but there is no assurance that using a medi kit will heal an old injury. You must now pass the check, and your ability to do so will depend on your Medicae passive. It is therefore wise to use your character with the highest medicae value when you start healing because they will have the best chance of recovering old injuries and will also heal for the most when you use those medi kits.
Simply going to your ship and resting there is another approach to heal your injuries and traumas. All of your group's wounds and traumas will be healed and reset when you get to your ship and that's how your injuries and traumas work in Warhammer 40K.
In Warhammer 40K, a wound is not referred to as health or HP. All of the things you are familiar with are called wounds and it is the number of wounds that you can take before your character goes unconscious. So, if a person has 34 wounds, it indicates that he can take 34 wounds before dying. Moreover, different attacks will cause a person to take different amounts of wounds, and once he heals, his wound tolerance increases. If you lose all of your wounds your character goes unconscious and they will be revived automatically at the end of battle.
Another great thing is that your character's health returns to maximum at the end of each combat. So, if your character is running low and you plan to finish the battle before he dies, don't worry about trying to revive him—that would be a waste of your medi kit. Instead, finish the battle and eliminate every enemy. After the battle is over, all of your characters' health will return to full, and if you know you'll be returning to your ship, it might even be worth it.