Direct answer: Kiln-made tar is tar produced by heating wood, bark, or other organic material in a kiln or tar-burning structure with limited oxygen.
Traditional tar kilns and tar dales were designed to control heat and airflow. The maker wanted the wood to break down and release tar, not burn completely into ash. In Scandinavian pine-tar production, resinous roots and wood could be carefully stacked so tar drained to a collection point.
Kiln-made tar is associated with pine tar, charcoal burning, historic woodland industries, Viking and medieval reconstructions, and rural craft. It is also a useful phrase for explaining the difference between open burning and controlled pyrolysis.
Kiln-made tells you about method, not tree species. A kiln can produce different tar depending on whether the feedstock is pine, birch bark, hardwood, peat, or another organic material.
In short: Made in a kiln or controlled burning structure. Depends on low oxygen and managed heat. Often linked with traditional pine-tar production. Method does not identify the exact tree or chemical composition.