Medium Armors are one of the three armor types in ESO along with Light and Heavy Armor. Medium Armor is generaly made from leathers or hides. Medium Armor is preferred by characters who have a rogue or thief playstyle. The Medium armor skill lines has passive skills that increase Stamina Regeneration as well as other bonuses.
Most abilities within this skill line scale with the amount of medium armor equipped. Each armor piece the player is outfitted with directly affects the boost gained from a passive ability; for instance, the passive ability labeled Wind Walker increases stamina regeneration. The stamina regen gained from Wind Walker is based on how much medium armor the player is wearing. If the player wears more medium armor they will receive a higher boost of stamina regeneration.
Players wanting to exploit these abilities should look to the racial section during character creation, specifically the Khajiit. The Khajiit possess a racial passive ability that allows the player to increase their experience gained within the medium armor skill line by 15%. Incentives to combine skills such as these are found with all races, so if you have not already created a character it would be helpful to study up on some of the racial skills and see how they correlate to other skill lines. (see races, to learn more about each of the 10 playable races and their abilities).
It is important to note that triggering the active ability within this skill line will consume stamina. Diversifying attribute costs (stamina, health, magicka) when choosing active abilities will help balance consumption. Maxing the most commonly used attribute first and leveling passive abilities that reduce cost can also help prevent hasty depletion.