Hithlain was a strong substance used by the Galadhrim of Lothlórien, which was grey in colour and felt silky to the touch. It was known to be used in the making of ropes, which were flexible and light, yet extremely robust. It may also have been used in the creation of the grey Elven cloaks of Lórien.
When Frodo and Sam tied their Elven rope around Gollum's ankle he screamed in pain. Apparently the material, or as Gollum claimed, the fact that Elves made it caused him agony when it was tied to him.