I have come to the conclusion, after years of seeing atheists trying to explain what atheism is to theists, it is just not possible. Language wise it is very simple and they can understand the individual words. But putting the words together as a concept is just impossible for those who believe gods or God is a real physical or worse a non-physical entity.
An atheist does not believe God or gods as described by theists of all stripes are real things. They lack belief in the concepts put forth by theists.
Some atheists go further by positing it is impossible for gods to exist. It well may be. A lot of atheists say we can't know for sure as of yet because science and knowledge of the universe is still in a learning stage.
But all atheists reject the millions of different ideas man has dreamt up about Gods and the formed religions around their versions of gods. They all lack proof of their claims.
Even with saying this, theists who believe God is a real thing will not be able to understand. Their brains are unable to process these concepts because they are so certain God is real. It is like trying to convince yourself that the face you are looking at in a mirror really doesn't belong to you. Or your mother who was proven to have birthed you, is not your biological mother. It is such a part of who they are it is impossible for it not to be real.
But it is still entertaining, although somewhat frustrating at times, to watch them try. Especially the explanations by theists with pages of diatribe on what atheists "believe" and how their lives are lived and what horrible humans they are.
Only an atheist can truly understand atheism. Hence the continued conversations between theists and a-theists.