The ban on deportation is an exemption for refugees and individuals seeking protection who cannot be officially recognised as refugees and who are at the same time neither entitled to asylum nor eligible for subsidiary protection. The ban on deportation comes into force if the journey back to the country of origin could seriously endanger the health or life of the person concerned, for example due to a serious illness.
Information on this can be found on the BMI website (under "Abschiebungsverbote")