Digi-Soul is the DMO equivalent of mana/energy/rage found in other MMOs. In DMO there is a tamer digi-soul and a digimon digi-soul. For tamers the digi-soul is used to digivolve to higher levels and maintain that level. For digimon it is used to perform the special attacks.
There are two ways to restore digi-soul, with one being usable in combat. First is to use vitamins out of combat that will restore both tamer and digimon digi-soul. The second is to use a DS disk, which is able to be used in combat as well as out. Digi-soul will naturally replenish out of combat and only when the digimon is in it's base form, else the tamer's digi-soul will slowly drain and the digivolved digi-soul will stay where it is at.