The lack of belief in a deity or deities. Just that. Nothing else.
Atheism doesn't hold any answers to life's big questions. It won't tell you the meaning of life and it won't tell you how it all started. It's not a religion, not a philosophy, not a way of life. Atheism is quite simply *non-belief*.
#1: "I heard you're an atheist and don't believe in God, #2. If that's true then how did the universe begin?"
#2: "I don't know for sure. The leading scientific theory right now is the Big Bang theory, but that has a few problems and might not be correct."
#1: "If there's no God then where do we know right from wrong from?"
#2: "Again, I don't know for sure, but the general theory is that we evolved this feature because social creatures do better if unit cohesion isn't hindered by constant murdering."
Atheism isn't about answers. It's not really about anything, actually.