"Healing" is a special ability unique to Journeymen and Heron Guards which restores a unit's health. Most damage a unit receives can be healed but some magical damage can not be fully healed.
Healing is normally done with the help of magical Mandrake Roots. Purple flowering mandrake plants can be found on some maps which Heron Guards and Journeymen can dig up to harvest the roots. One root = one unit healing.
While healing restores a light/good unit's health, if lesser undead units are healed they are destroyed (wights, thrall, etc).