The Masters of the Mystic Arts are the most skilled magic users in the universe, 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 are commonly known as the "Masters of Kamar Taj".
When the cosmic being Dormammu threatened Earth, Kaecilius betrayed the Masters of the Mystic arts and allied himself with Dormamu as his herald. As the Masters of the Mystic Arts began their battle with Kaecilius, many were killed in the process.
After Stephen Strange defeats Kaecilius and Dormammu, he fullfills 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 during the Time Skip. After the Blip (aka the UnSnap) happens, these new Masters (alongside Strange and Wong) help defeat Thanos, the Outriders, the Chitauri and the Black Order.