What is the purple ropes on naruto characters?

Seals, also known as fūinjutsu (封印術, lit. sealing techniques), are techniques used to either store objects inside a person, trap or release people or entities into or from containers, suppress abilities, prevent flow of fluids in a target and so on.

Naruto himself can create the Eight Trigrams Seal (八卦封), derived from Kushina Uzumaki's Adamantine Sealing Chains with his Sage Mode chakra while in Nine-Tails Chakra Mode: Ashura Mode, to bind Kaguya's body after separating her from Black Zetsu and subsequently destroy her entire being via Sage Art: Ultra-Big Ball Rasengan, thereby lifting the Ten-Tails' resurrection.