So the Buddha was an ordinary person who learned something of profound truth from his efforts to understand what is true. This truth is described as Bodhi, the knowledge of the truth of existence. That is quite a clunky way of putting it but there is a difference between understanding something and knowing it. When you know something it informs your actions when you understand something it can be intellectualized and practically ignored. This is a big problem today when we understand that our fossil fuel burning is almost certainly bringing about serious damage to our environment yet we do very little to stop it.
What was it that the Buddha knew? This is also simple and the consequence of the Buddha’s insight that is known to us as “anatta”. Anatta means “no atman”, atman was a concept developed between 1000 and 500 bc (ish) which can loosely be described as self, soul and god. So the Buddha knew that there was no self, no soul and no god or gods. He also knew that there was something beyond self,and beyond material. Language is not very good or capable of describing this but it is not necessary as you are it, but so is everyone and everything else. One way of putting it would be that we are all the same.
By the way the atman project is what the Hindu upanishads are about and could be described as an attempt to provide a more accurate model of existence than the polytheistic one of the Vedas. Unfortunately the project was skewed by the need for the Brahmanas (the Hindu teachers) to keep their business model going. That stopped them from reaching the same quite obvious conclusion which Buddha did. We have always found it wise to look out for ‘business models’ behind any spiritual or ideological initiative and to see if it skews the message.