Bhairava is one of the forms of Shiva, said to be fearsome and formidable. The consort of Shiva in this case is called Bhairavi (Shakti)… that is, the power that manifests this particular aspect of existence.
There is a distinct sect of tantra who worship this aspect of Shiva and Shakti. They are called bhairavis. There is also a well-known tantric text called the Bhairava Tantra.
For the purpose of meditative practice, this is a particularly comfortable mudra. What is easier than placing the hands one on top of the other in the lap, while sitting in a meditational asana? It is a mudra that people do almost automatically.