A hand file is essentially a hand saw with a very wide blade. Just like a saw, it cuts material using teeth. A typical hand file is made from a bar of high-carbon steel with its teeth pressed, cut, or raised into the steel. After the teeth are made the bar is heat-treated making it harder than most other materials it is likely to come across. As a result, when those teeth are rubbed across another piece of softer material, the teeth will dig into the material and pull bits away.