Dog owners who live in the city need a leash, a rope or cord attached to the dog's collar that helps his owner keep him safe and restrained.
When you walk a dog, you hold his leash to guide him along the sidewalk and keep him from chasing squirrels into busy streets. The word leash is also useful for describing a figurative kind of guide or restraint: "The teachers at that school keep their students on a short leash when they take field trips downtown." The students aren't on actual leashes, but they are constrained. Leash comes from the Old French word laissier, or "loose."