Direct answer: Rope tar is tar used to dress, preserve, and weatherproof natural fibre rope, especially hemp or similar cordage used on ships and outdoor structures.
Natural fibre rope is strong but vulnerable to moisture, rot, abrasion, and weather. In maritime use, tar helped extend its working life and made rigging more resistant to harsh conditions. The smell and darkened colour of tarred rope became part of the sensory world of wooden ships.
Rope tar appears in sailing history, reenactment, rigging, craft cordage, and museum conservation. It is usually associated with pine tar or wood tar mixtures suitable for rope rather than coal tar or roofing tar.
Rope tar should be distinguished from tarred rope as a finished object. The tar is the dressing or preservative material. The rope is the fibre structure that receives it.
In short: Used on natural fibre ropes. Historically important in ship rigging. Usually associated with pine tar or wood tar. Not appropriate as casual treatment for safety-critical modern rope.