Paint thinner is a solvent used to reduce the viscosity of oil-based paints, varnishes, and stains for smoother application. It also cleans brushes and tools after oil-based work, since water won’t cut it.
The right thinner depends on the product being used. Mineral spirits handle most oil-based paints, while lacquer thinner suits faster-drying finishes, and mixing the wrong one ruins both the product and the tool. Labels tell you which solvent each coating expects, so that the correct thinner is applied to the appropriate job type.