Armor of the Trainee is a light, medium or heavy armor set. The 5-piece bonus increases your Maximum Health, Maximum Magicka, and Maximum Stamina by 1250.
Combined with the two, three, four and five piece bonus you can gain a lot of extra resources from this set, which can be nice. The Trainee set was designed for leveling because the original trait provides a bonus to XP gain and it gives you a nice bonus to all of your stats. However, the set was also later seen in some end-game PvP builds due to the stat bonus.
The Armor of the Trainee set is part of the base game and drops in the starter zones Stros M'Kai, Khenarthi's Roost, Bleakrock Isle, Betnikh, and Bal Foyen in the Overland content. You can also find it on Guild Traders. Each set item is bound on equip. You can acquire heavy, light, medium armor, jewelry and weapons of this set in the Redguard, Nord or Altmer motif style (depending upon which alliance zone it drops in).