Atheism is about belief, agnosticism is about knowledge.
They are complementary.
You can be atheist/agnostic, theist/agnostic, atheist/gnostic, theist/gnostic. It's a matrix.
Also there are two flavors of atheism: those that believe there is no god (a minority in my opinion, and that position requires a burden of proof).
And there are atheists who simply reject the claim that there is a god. They say there might be a god, or maybe not, but that people who claim there is a god have not presented enough evidence for that claim to be accepted.
If I look at a grass field and tell you there is an even number of blades of grass in that field, you are not going to accept my claim.
Does that mean you think that number is odd?
No. You're just saying I have not provided evidence for my claim, and therefore, you do not accept it. But you are not taking any position yourself.
That's atheism.