When Jomsvikings are wounded, they will use their own rations to heal up first, but once they are out of rations they will go down.
You then have a certain time window to get to your Jomsviking in time to revive them with one of your own rations.
If you are unable to your fallen comrade, they will fall in in battle and be unavailable for future raids for some time.
You should also keep in mind that losing all Jomsvikings during the raid will result in you not being able to force open doors or chests, so keep an eye on your raid party.
Feel free to get in touch if you have any additional questions!