Heavy armor must involve solid metal plates of some sort arranged in a fairly rigid way. Metal scalemail or lamellar counts, provided that the armor doesn’t flex very much, as do metal brigandines and proper plate armor.
(Note: Brigandines or coats-of-plates, where plates of hard material are riveted to an outer covering of cloth or leather, must be made out of metal, and cannot be made out of hide, leather, or any other material, if they are to count as Heavy Armor.)
You can only wear Heavy Armor if you have the Myrmidon ability.
If you do not have this ability, you can still wear a few pieces of metal plate or lamellar (bracers, pauldrons, greaves, etc.) but they count as pieces of Medium Armor (if you have the Warrior Ability) or Light Armor (if you don’t have Warrior) for the purposes of calculating Hit Points and ignoring Cleave and Impale effects.
Wearing a suit of Heavy Armor increases your maximum HP by +3.
Pieces of Heavy Armor (even if they are not part of a full suit) grant protection from the Impale and Cleave Calls. If an attack delivering such a Call hits a piece of Heavy Armor, you are not affected by the Impale/Cleave effect. You still take 1 point of damage from the hit, however. A Cleave or Impale to a location not covered by a piece of Heavy Armor will deliver the Cleave/Impale effect as normal.
It’s common courtesy to call “Armor!” when someone strikes you with a Cleave or Impale that you can ignore because of your armor, to let them know why you are ignoring their Call.
If you do not have the Myrmidon ability, you cannot benefit from any of these effects.