From Tolkien Gateway

Hithlain was a strong fibre made from the inner bark of mallorn trees.[1] It was grey and felt silky. The Galadhrim of Lothlórien used it to make ropes, which were flexible and light, yet extremely robust. It may also have been used in the creation of the grey Elven cloaks of Lothlórien.[2]

Sam and Frodo climbed down a cliff of the Emyn Muil with the help of a hithlain rope, which Sam tied to a stump at the top. When Sam tugged the rope in annoyance that they'd have to leave it and it would help Gollum get down, it came loose and fell on him. Frodo thought Sam had tied it badly, but Sam thought it came because he called it.[3]

When Frodo and Sam tied their Elven rope around Gollum's ankle he screamed in pain. Apparently the fact that Elves made the material, or as Gollum claimed, caused him agony when it touched him.[3]