A leash is a rope, chain or strap attached to the collar or harness of an animal or person for restraint or control. Leashes are probably best known from their use for dogs. They are available in any pet shop.
Leashes in BDSM
In the BDSM context, a Master or Mistress may place a collar on their slave. Collars denote ownership of slaves, perhaps by placing the Master's name on the collar. A leash makes training a slave much easier. A training collar has rings for attaching a leash conveniently.
Leashes are also used in doggie training.