Leather armour refers to pieces of armour made of animal skins.While leather clothing was common and the typical vest or coat of rigid leather did exist and were worn, these were usually not considered real armour and they indeed offered little protection against sword cuts or thrusts and literally no protection against heavy blows.
Real leather armour was made of thick, boiled leather, many layered (often up to six layers or more,), hardened, laquered or glue coated leather or rawhide.Also leather-covered linen and padded armour did exist and of course leather was worn beneath or upon other types of armour such as mail or plate.
Though there is some evidence that early Cuirasses may have been made of Kurbul, the most common types of leather armour actually were either leather lamellar or leather scale armour.Also types of leather helmets existed as well as gloves, arm guards and vambraces, though most of these were intended to be worn in combination with mail or plate.
These types of leather armour were very sophisticated and effective and quite expensive,and not to be mistaken for a cheaper substitute for metal armour at all.
Corsairs of Umbar
The Umbareans sometimes wore leather-coated linen or leather armour consisting of fine plates of laquered leather.
The Drûghu sometimes wore coats made of rawhide for protection.
The Dunlendings often wore archaic but effective armour made of rawhide.
The Avari sometimes wore a sort of Cuirass nade of hardened, laquered leather-plates, or a type of pelt-padded armour they called "Esgel", the Eldar wore rigid leather, but mostly in combination with either mail or fishscale armour.Some Noldor or the the warriors of the mole-folk wore a type of harness made of leather-straps, the "lattar".The Falmari and Falathrim wore the "tampa" also a type of aketon or buff-coat consisting of padded material often covered with layers of leather.
The men of Gondor sometimes wore rigid leather sbove mail or armour made of leather-covered padded or linen armour.
The Haladin seem to have known types of hardened leather or leather-covered padded or linen armour.They rarely ever used mail.
The Númenóreans often wore beautifull leather-covered linen or metal plate armour or adorned rigid leather above mail.
Northmen of Rhovanion
The Northrons wore leather-jerkins, in reality most often either leather-covered linen or padded armour or multy-layered boiled leather.
The Rohirrim wore leather vests in combination with mail, but sometimes also used leather-covered types of padded armour or hardened leather alternatively.