Hobbles are devices used to restrain horses. They are most commonly used in wilderness areas.
Hobbles consist of two leather (or sometimes synthetic) "bracelets" joined with a chain or a leather strap - kind of manacles for horses. They secure the horse's two front legs together. The horse has enough freedom to move and graze, but can't actually run away. Horses have to be specially trained to tolerate hobbles (or they will panic and hurt themselves).
Hobbles are sometimes called hopples.
Picture shows horses grazing "loose" in the back country in Alberta, Canada. Several of them are hobbled.