In most cases, a "master" in the Bible refers to truth. That can vary according to the person being called master -- Egyptian masters respresent factual, worldly knowledge; when the Lord is referred to as master it means Divine Truth itself. The term does vary according to context. If the things directed by the "master" represent knowledge and ideas, the master will represent desires for good, since desires for good are the things that inspire true thinking.

To fast as in Mark 2:19, signifies to mourn on account of the defect of truth and good. Fasting corresponds to temptation.

In 2 Samuel 12:23, this signifies to mourn because of some lack, usually of truth and good. (Apocalypse Explained 1189[2])

In Matthew 4:2; 6:16; Mark 2:19; Luke 4:2; 5:34, this signifies affliction such as occurs in temptations. (Apocalypse Explained 730[41])

In Matthew 9:15, this signifies that they are in an unhappy state when good is no longer conjoined with truths. (Arcana Coelestia 9182[10])

(Odkazy: Apocalypse Explained 1189; Mark 2)