Tinder are Materials used to help in starting a fire. One tinder is always used when starting a fire, and it will be consumed regardless of whether or not the player succeeds. Tinder cannot be used to increase the length of a fire already started.
When the Skill Level for Fire Starting reaches Level 3, the player no longer needs tinder to start a fire. The player may still use tinder if they like, but since it provides no benefit, the only purpose would be to get rid of the item.
The most commonly found material for tinder is a Tinder Plug. These can sometimes be found in buildings. They may also be obtained from harvesting a cardboard box, which will give a few tinder plugs based on the size of the box (a Hunting Knife can help break down cardboard boxes quicker). Harvesting a cedar limb will also give the player a Tinder Plug in addition to several pieces of Cedar Firewood, at the cost of at least 45 minutes to chop the wood, as well as requiring a Hatchet. Two tinder plugs may also be crafted in the radial menu by using a Stick.