Wood tar isn't a substance we think much about these days, but in ancient times, it was critically important. Shipbuilders throughout history have relied on tar to seal roofs, waterproof sails, and glue together boat components. Unsurprisingly, the seafaring Vikings were especially proficient at making the substance, and it remained one of Scandinavia's most important exports through the 1800s. Wood tar also has anti-microbial properties, so it was often applied to wounds to prevent infection — as a result there's an old Finnish saying, “if sauna, vodka, and tar won't help, the disease is fatal.”
To produce wood tar, resin-laden wood (such as evergreen fatwood) must be heated in an enclosed dry still or kiln. This process melts the resin deposits, which run down into a collection vessel beneath the oven. Some of the deposits will result in sticky tar (also called pitch) while others will create a thinner oily substance (wood oil or turpentine). It also leaves behind charcoal, which we all know is valuable as a long-burning fuel source.
Traditionally, tar was produced in tar kilns made from limestone, clay, and soil as pictured above. (Source: translation from Bergström 1941: part II, p. 57; CC BY 4.0.) However, it's also possible to achieve similar results using a few modern materials. Russian survivalist Max Egorov of the YouTube channel Advoko Makes showed how he made a wood tar kiln (or still/distiller) using a metal oil drum, a copper pipe, and some natural clay.
This setup generated several mason jars full of oil, as well as some globs of thick tar and a large amount of brittle charcoal. Max used the tar to glue legs onto a stool he made, and finished the wood with the oil. However, these materials could easily be used for sealing a boat or even fueling an oil lamp. If you're in an area where fatwood or other resin-heavy woods are present, this tar kiln method is a good primitive skill to know.