Not all forms of Buddhism denny the existence of Gods, heavens, hells, re-incarnation, or other supernatural experiences. Atheist Buddhism emphasizes the parts of Siddhartha Gautama's teaching that emphasized focusing on compassion and effectiveness in this world for this world. Further, for the Atheist Buddhist, unlike many Atheist Westerners, the highest quality of life, or enlightenment, comes not from glorifying the self but by doing things that allow us to transcend ourselves to where we realize the self is an illusion and the barrier between "I" and "reality" disappears. When you are truly in the moment, in that "just do it" mindset, we do not know or care about the past or future, but just "are we doing the compassionate and effective thing right now?"
Not every Buddhist practices an atheist buddhism. Some believe in re-incarnation and some believe in a heaven/hell structure.