Q: What is moksha and how can this state be described?
A: From the perspective of Advaita Vedanta, moksha is the direct realization of the fact that 1) You and the universe/God are non-separate from one another and 2) You and the universe/God are fundamentally identical as brahman, the one true reality. To use a common Vedanta metaphor, this realization is like a wave first understanding that it is non-separate from the ocean and then realizing that it is fundamentally identical with the ocean as water. Here, the wave represents you, the ocean is the universe/God and water is brahman.
So moksha is realizing “I am brahman” (Brihadaranyaka Upanisad 1.4.10). Since brahman is “defined” for instance, as “that which has no sin, no decrepitude, no death, no sorrow, no hunger, no thirst…” (Chandogya Upanisad 8.7.1) then realizing that you are—and always have been—brahman means that YOU are free from birth, death and suffering. This is moksha i.e. freedom (moksha literally means “liberation” or “freedom” in Sanskrit) and it is synonymous with enlightenment (self-knowledge) in Advaita Vedanta.
Enlightenment in this sense refers solely to the direct realization that you are the ever-free brahman. Since you are brahman and always have been brahman, this is just the recognition of an already existent fact, not the attainment of a particular state. By extension, this also means enlightenment is not becoming brahman or merging into brahman. Why? Because you can’t become or merge into the brahman you already are, similar to the way that water can’t become or merge into the water it already is. You can only recognize that you already are brahman and that you’re already free.
All my best – Vishnudeva