Mail is a protective iron fabric made up of thousands of interlocking iron rings. Mail was made with a 4-in-1 pattern, where each ring passes through four of it's nearest neighbors.
During the Viking age, chainmail usually was worn in the form of a t-shaped shirt or brynja. The garment had short sleeves (half to three-quarters length) and it's length reached the thighs. A common chainmail had a weight of around 10-16 kg. This weight is not particularly burdensome, since a lot of the weight is taken up on the hips.
Warriors wore padded garments under their mail to help absorb the force of a blow. Typically, these garments consisted of two layers of wool or leather stuffed with fleece or animal hair, which were then sown together.
Scale armor or spangabrynja was made by attaching many small overlapping plates of metal or horn to a flexible backing made of leather or fabric.
Source: Hurstwic: Viking Mail
Chainmail has strong defensive advantages. It has a protection of 2 tot 3 points, but it hinders the movement of a character which is lowered by 1 point. Chainmail is pretty pricy so not all warriors can afford one in battle.