The Masters of the Mystic Arts are an organization of skilled magic users, a loyal order of sorcerers and sorceresses dedicated to protecting the Earth from magical and cosmic threats. While each individual Master has his/her own headquarters, a Sanctu Sanctorum, together the Masters of the Mystic Arts reside in Kamar Taj, hence why they're commonly known as the "Masters of Kamar Taj".
Kaecilius, who was once a member of the Masters of the Mystic arts, betrayed the organization to seek immortality and formed his own group of sorcerers called Zealots. He attempted to bring the Dark Dimension and its ruler, a cosmic being named Dormammu, to Earth. The Masters of the Mystic Arts fought Kaecilius and the Zealots, which caused the death of many sorcerers.
When Dormammu threatened Earth, Stephen Strange is able to stop and convince the cosmic being to take all the Zealots with him, banishing all of them to the Dark Dimension and saving the Earth. With this, Strange fulfills his duty and his destiny to protect the universe from magical and cosmic threats, with Wong at his side and both using the New York Sanctu Sanctorum as their headquarters.
Many new people have joined the Masters of the Mystic Arts and became sorcerers since then. After the Avengers brought back half of all the life that was erased by Thanos, these new Masters (alongside Strange and Wong) help to defeat the Mad Titan, the Outriders, the Chitauri and the Black Order.
Krugarr is a former Master that left the organization to join the Ravagers.
