A handcuff is a metal restraint that closes around a person's wrist. When police arrest a suspect, they often put handcuffs on him.
You'll almost always find this word in its plural form, since handcuffs come in pairs, one for each wrist. It's also a verb, meaning "to put handcuffs on a person." Police officers carry handcuffs with them, so that they're ready to handcuff criminals and troublemakers. Handcuffs hold the wrists close to each other, usually behind the wearer's back. The earliest meaning of handcuff was "decorative sleeve," in the 1640s.