Medi-packs (also referred to as Narthecium) allow a unit with a medic to ignore the first failed saving throw it rolls in every turn.
The medi-pack may not be used on any model who's suffered 'Instant Death' (see page 53 of the Warhammer 40,000 rulebook) or that has been hit by a close combat weapon that allows no save at all. The medi-pack may not be used if the medic is in base contact with an enemy model.