Medium armor can be any kind of inflexible leather or rawhide, including leather scales or lamellar, or flexible metal armor such as chainmail or loosely-attached scalemail.
You can only wear Medium Armor if you have the Warrior ability.
If you do not have this ability, you can still wear a few pieces of hardened leather or chainmail (bracers, pauldrons, greaves, etc.) but they count as pieces of Light Armor for the purposes of calculating Hit Points and ignoring Cleave effects.
A suit of Medium Armor increases your maximum HP by +2.
Pieces of Medium Armor (even if they are not part of a full suit) grant protection from the Cleave Call. If an attack delivering such a Call hits a piece of Medium Armor, you are not affected by the Cleave effect. You still take 1 point of damage from the hit, however. A Cleave to a location not covered by a piece of Medium Armor will deliver the Cleave effect as normal.
It’s common courtesy to call “Armor!” when someone strikes you with a Cleave that you can ignore because of your armor, to let them know why you are ignoring their Call.
If you do not have the Warrior ability, you cannot benefit from any of these effects.