What is meant by "Brahma Satya Jagat Mithya" of Advaita Vedanta?

According to Advaita Vedanta, Enlightened person (Jnani) behaves exactly like a normal person (Ajnani).

The main difference is Ajnani associates “I” with Jagat which includes this world, name, body, mind, ego-self and is ignorant of the true Self. A Jnani associates “I” with the core of one self which is nameless, formless, Concentrating Entity, Consciousness, Awareness, Chit Shakti (Sentient Energy), Atman (Self). A Jnani knows that the real “I” is completely distinct from the body and the mind.

When one plays a role in a Drama, no matter how much one gets into the character played in the Drama, one does not forget one’s real name and self and effortlessly knows all the time that it is just a role being played in the Drama.

Exactly in the same way, a Jnani knows that in reality we all are Atman (Self), nameless, formless, Consciousness and the name and the body we have is just to carry out the role each one has in this world and carries on with the given role with full responsibility.

But an Ajnani completely identifies oneself with the name, body, mind and thoughts and carries on with the responsibilities thinking that the external world, body and mind are the only real things without being aware that in reality he is Atman (Self) and not the body and the mind.

I have shared details on how exactly to get enlightened and become Self-Realized in my answer to the question How can I attain Self-Realization according to Ramana Maharshi?