Tar is water-resistant and can help shed water, but it is not automatically a perfect waterproof membrane. Its performance depends on the type of tar, surface, thickness, weathering, and maintenance.
Tar has a long reputation as a waterproofing material because many tars repel water and cling to surfaces. On wood, rope, canvas, roofs, and seams, a tarry coating can slow water entry and reduce wetting. That is one reason tar became so important in maritime and outdoor work.
The word waterproof can be too strong, though. A tar finish can crack, soften, wear away, collect dirt, or fail at joints. On rough timber, it may protect the surface while still allowing movement and moisture exchange. On a roof, the exact product and installation method matter far more than the general word tar.
Traditional Stockholm tar and pine tar were often used as breathable, renewable treatments for wood and rope rather than as sealed plastic barriers. That is part of their appeal in heritage work. They can be maintained and reapplied, but they do not behave like a modern synthetic membrane.
The direct answer is that tar is usually water-shedding, sometimes water-resistant enough for traditional use, and not something to trust blindly for modern waterproofing. Identify the product and the job before relying on it.