The wikipedia defines ascended masters as spiritually enlightened beings who in past incarnations were ordinary humans, but who have undergone a series of spiritual transformations originally called initiations. (https://en.wikipedia.org/wiki/Ascended_master)
The term comes from Theosophy, but the concept itself is found in several ancient wisdom teachings, as well as modern New Age belief systems.
Ascended Masters are spiritually more advanced souls, usually of humanoid origin, that have reached higher levels of consciousness and transcended "life in a three dimensional reality." They even transcended the cycles of life and death, and have become immortal. Once they attain these levels of consciousness they have the choice to remain individuals, to become part of a collective, or to merge with others and become part of a pool from which new souls are born. If they remain individuals or join the collective(s) of ascended masters, they remain dedicated to service by teaching in many worlds. Many spiritual teachers that have walked the Earth, are believed to be ascended masters. Jesus Christ, Krishna, and the Buddha are but some of them. They all came with teachings that were meant to uplift mankind, to enhance man's awareness, and by doing so, raise man's levels of consciousness.
The word 'ascended' refers to the Ascension process, which some describe as 'the road back to God,' and others as the road to enlightenment. Ascension consists of going up one step on the density or dimension scale.