A harness is a gear made of straps, metal rings, buckles, rivets, loops, ropes etc. that goes around the body. It is usually adjustible such that it fits tightly when put on, similar to the way a belt is worn.
Harnesses in BDSM
In BDSM, harnesses have an iconic status and many BDSM practitioners and fetishists love to wear a harness, or to see a harness worn by their partner. Harnesses are worn by both genders, and both by submissives and dominants. They come in many different designs and are often very individual. They are usually made to be adjustible within certain size limits, but you can not assume that a harness that fits one person will also fit another person. Individual body measurements tend to differ quite a lot and so harnesses for people are often custom made and rarely purchased off the shelf.
Harnesses are worn as a type of fetish clothing. When worn by subs, they may also serve as as a means of restraint. They are popular in pony play, but also in slave training and many other cases. Harnesses in BDSM and fetishism are often made of (stereotypically black) leather, but may also be made of PVC or metal, e.g. with chains or in armour style.
A bondage harness is a harness designed to serve as a restraint. It is always close fitting and cannot be removed without undoing one or more buckles.
There are also harnesses for strap-on dildos.
Harnesses outside BDSM
Outside BDSM and fetishism, harnesses are often put on animals, e.g. horses, cows, camels, or dogs. Harnesses for humans are used in sports such as climbing, diving, skydiving, bungee-jumping etc. — whenever something must be securely attached to a person, withstanding weight and hard pulls. There are also child harnesses, which are safety restraints for walking with small children, or for the safety of a small child on a car seat.