Have we ever asked ourselves why human being always interested to know their cause? From wanting to know one's own ancestor to tracing back what kind of living organism we are evolved from. Seems like by knowing the cause of the universe, we can handle our well being in a better way. And for vedānta students, why do we need to know brahman which happened to be the cause of the universe? Because śruti herself said that by knowing brahman one will be free from the life of becoming. And this cause is not another entity which is different from me, not as we usually think that the creator as the intelligent cause is always different from the creation, and in the same time the material cause is also needed. Sat brahman is the intelligent and also the material cause of the universe. The means - sadhana is knowledge of brahman and the end - sadhya is freedom from samsāra. If by knowing this limitless brahman, one will gain limitlessness, the only possibility is that brahman is me alone. Gaining what is already gained is possible only with knowledge - jñānam. All along I have wrongly taken body-mind-sense-complex to be me, but they are only attribute which is subject to change, and anything subject to change is not svarūpa - true nature. My svarūpa is sat - existence alone, without which I am not.
"Entire universe is having its basis on sat - existence alone", this is the pratijñā - conformation of the teaching. Let it explained through examination on a clay pot. Does "pot" an object exist by itself or does the object indicated by word "pot" depend upon clay to exist? Clay, because of which pot comes, exists, and unto which it will go back to, is the abode - adhiṣṭhana or another word kāraṇa - cause of the pot.
Usually when we say "pot is" means "pot existence is", and "pot is not" means "pot existence is not". But vedānta says "pot is" means pot existence is, and "pot is not" means "pot existence still is". Because existence has to be there for any name and form to manifest. What is called to be nature is such that can't be ceased away in all period of time, means there is never a time existence is never exist, thus existence brahman is said to be unborn - ajam. But will our problems solved when we know our true nature is existence? If we couldn't see it, how about we try to see the fact that all the attributes which we think is me or belong to me, this body-mind-senses which is subject to disease, old-edge, likes-dislikes, sorrow, etc, all are mithyā which is bound to constant change and always depend on me existence to be exist. We have tried so hard to be a person or in the condition which we have the most satisfaction, but at the time we are having it and enjoying it, at that time itself it is having modification again. In this case how can we get full satisfaction in our life which there is no sorrow and only happiness? It just like a person being missed diagnosed with terminal disease and struggle to cure, how released would be for him/her to know that all along it is just a miss-diagnosed, the disease wasn't really there all time. Just like Lord Krsna say to Arjuna "you are grieving over those who are not to be grieved for" (Gītā chapter 2.11), body is going to die, you know about it clearly, but you still grieve about it. Not just what we are taking to be me and mine are mithyā, in fact the entire world which is the potential to relate with me is mithyā alone. I am alone is satyam.
Therefore existence brahman is the sustainer of everything - sarvabhṛt, yet totally free from them asaktam - totally detached, physically and emotionally. We get attached to the world through the body-mind-senses, but ātmā is free from body-mind-senses. Since I am free from this body-mind-senses, it has no way of getting attached to any other things. Just like the rope sustains the snake, in the sense that without it there is no snake, in the same manner mithyā is dependent upon satyam. But why the image of the snake can appears on the rope? It is explained with māyā sakti - the power of āvarana - covering and viksepa - projecting.
Now the problem arises, since māyā which consists of three guna (sattva, rajas, and tamas) has its basis in brahman, then why not brahman also have three guna? To negate this, Lord Krsna says brahman is nirguṇaṁ - free from guna. With trigunātmikā māyā, there is no doubt that brahman is giving the status as the cause of the creation. But it is just an incidental attribute which attributed to brahman. The svarūpa - nature of brahman is satyam jñānam anantam (existence - knowledge - absolute). Māyā the modifying cause for the mithyā creation is also mithyā and thus brahman remains free from all guna. At the same time, brahman is the experiencer of guna - guṇabhoktṛ ca, where objects of experience and organs which perceived are created by guna. "Guna gunesu vartante" - all the transaction are only guna contact with guna (Gītā chapter 3.28). Since brahman is the basis for those contact to be exist, therefore brahman is said to be the experiencer as though.