Ascended masters is an understanding from theosophy that has also been adopted by some of the new age, and neopagan movements. Ascended masters are people who reached a form of enlightenment, or ascension during their physical incarnation, and because of that have been blessed with wisdom, and knowledge. They are also sometimes referred to as Mahatmas meaning “Great Souls” in Sanskrit. Ascended masters are people who have learned all the lessons they have needed to, and have completed their divine plans, and because of this they have become connected with there Divine selves and because of those have become God-like. A lot of people who took on the understandings of ascended masters have very close, but different understandings of them, which has resulted in many different people being considered ascended masters. Such people as Jesus, Confucius, Gautama Buddha, and St. Germain, along with many others are considered to be ascended masters. In theosophy, and in some new age understanding the ascended masters are connected to, and a part of a group of entities that are known as the Great White Brotherhood.
From my understanding I personally consider anyone who has gone through enlightenment, or ascension to be considered an ascended master. I feel by working with these entities you are able to learn their teachings, and will have a nice guiding hand to reach your maximum potential. Ascended masters are able to teach from the higher planes of existences, and can be connected with much like all other entities. Incarnated ascended masters will usually share their wisdom and knowledge with those who will listen. The ascended masters have self realize their connection to divinity, and have become the highest forms of themselves. They have great wisdom, and understanding, which many different people can learn from to better themselves and their practices. Everyone has the potential to, become an ascended master, and so many people throughout history have achieved this enlightenment, and ascension in their own ways.