Cross ties are two straps that are secured to either side of a barn aisle. The horse is then tied to both ties. This effectively immobilizes the animal - although many horses are comfortable just standing there once they are trained to cross ties. (Some horses, however, never learn to tolerate them).
Cross ties are popular in the United States but seldom used in the UK. They're commonly used in riding schools to hold a horse steady while novice students learn to groom and care for a horse. Show people also use them when doing extensive preparation for a conformation or model class, and I've used them to put a costume on a horse. Racehorses are also often put in cross ties. Finally, cross ties are sometimes used to hold a horse that can be difficult when being groomed and tacked up - they will prevent a horse from being able to reach to bite. (Some people prefer to use a muzzle instead).
Chain cross ties are sometimes used, but are considered dangerous by others because if something goes wrong, the chain may whip around and hit the horse or a handler.
This draft horse is in chain cross ties. The heavy boots are for protection while shipping, so he just got off a trailer. Image source: Pitke via Wikimedia Commons.