Direct answer: Tar varnish is a dark protective coating made with tar, pitch, oil, resin, solvent, or similar ingredients, depending on the historical recipe or product.
Before modern coatings, makers mixed available sticky, oily, resinous, and tarry materials to create protective finishes. A tar varnish might be designed for wood, iron, rope, or outdoor surfaces. Recipes could vary widely by trade and period.
Tar varnish appears in old boatyard practice, ironwork, farm equipment, timber preservation, theatrical aging, and heritage craft. Its appeal is a dark surface, water-shedding behaviour, and historical character.
The phrase does not guarantee a transparent furniture varnish. Many tar varnishes are dark, smoky, and practical rather than decorative in the modern interior-design sense. The exact ingredients matter greatly.
In short: A coating category rather than one formula. May combine tar with oils, resins, pitch, or solvents. Often dark and practical rather than clear and decorative. Commonly appears in historical craft and outdoor-use contexts.