This refers to a Dominant using a belt to spank/whip their submissive/slave.  It is also referred to as 'STRAPPING'.  It is usually made from a strip of heavy leather, often with a handle. A broader toy made from leather is usually referred to as a paddle, but these can be made from other materials.  Belt spanking/whipping forms part of the BDSM activities that involve and include Impact Play. The forceful impact a disciplinary strap/belt makes can easily knock a submissive/slave off balance and as a result, most Dominants will usually tie or restrain their submissive/slave down before beginning this type of play. In the case of 24/7 slaves, belt spanking/whipping is often used in the BDSM scene as a form of discipline, and depending on the person, can range from light to heavy play that can result in the submissive/slave being bruised or bleeding.  

A belt spanking/whipping is far more painful than a bare hand, especially when it is across a bare bottom/backside or a bare back. It can also leave welts that can take a lengthy period of time to heal. It is an unbearable searing pain, especially if the hits come rapidly one after the other so that the highest intensity peak of pain from one lick is immediately followed by the next lick so the fire blazes hotter with each bite.  Masochists (people who enjoy receiving pain), range from those who can take light taps to those who want their skin broken and bleeding.  There is no judgment and for anyone outside of the BDSM/Fetish/Kink community and lifestyle, it may seem barbaric, bizarre, and completely pointless. 

The key to BDSM activities like these is that there is always consent, respect, trust, and an agreement between a Dominant and a submissive/slave for this type of play to take place.