A chain is a series of interconnected metal links. It is often used in bondage scenes. Notably, it is different to rope in that it is cold and unyielding. Chains do not stretch and so cannot typically be a snug fit. Thus, a bottom restrained with chain will usually have some degree of movement or wriggle room available to them.
Due to its lack of give, chain isn't very useful for complete physical restraint---such as to stop all movement---but it does work well to confine movement to a limited range, such as to limit the range of movement of arms and legs in handcuffs and leg cuffs, or when used in a leash.
Generally speaking, chains cannot be knotted in the same manner as rope. Instead, padlocks are commonly used to join or anchor the ends of chains.