Direct answer: Viking tar is the modern name often given to the wood tar used in Viking-age boatbuilding, timber protection, and possibly other craft work in Scandinavia.
Scandinavian boat traditions depended on wood, fibre, iron, and tar. Archaeological and experimental work has shown that tar production could be organised at meaningful scale, especially where boatbuilding and seafaring created heavy demand for waterproofing and preservation materials.
The obvious use was boat care: coating, preserving, and maintaining wooden hulls, ropes, and seams. Viking tar is also relevant to reenactment, shield and chest finishes, house timber, and the reconstruction of early medieval Scandinavian technologies.
Viking tar is normally pine or other wood tar used in a Viking-age context. It does not mean a unique recipe known from a single surviving manual. The exact composition would vary by forest, kiln, and maker.
In short: A modern phrase for tar used in Viking-age contexts. Usually connected with Scandinavian wood tar and boatbuilding. Relevant to archaeology and experimental reconstruction. Not one fixed, standardised recipe.