Chains are a material created by forging smaller links together out of a durable, metal material. A chain is similar to rope in the sense that it is surprisingly flexible, although it is notably more durable and less smooth. Chains are virtually impossible for a human being to break on their own, and thus are often used in more extreme bondage to prove that the subject is throughly restrained. In spanking, chains are rarely used to bind a spankee (as if often seems too intense), although chains are a part of handcuffs and shackles, both found in spanking.
Vintage French photo of a slave in chains from Biederer Studio (c. 1930s).