The firestarter is a tool in Stranded which can be used ignite resources. The resources' ignite temperature is often lower than it's self light temperature. However, the heat produced by a firestarter is lower than tools who light resources by heating them up, to their self light temperature. Thus, a firestarter in combination with a heat producing tool like a Firemaking Bow is a great combination to light fires.
To create a firestarter, two stones are needed: a Flint and a Pyrite. Pyrite is an ore with a high amount of Iron in it. Iron and flint produce sparks which can be used to light resources, also in real life.