Generally, atheism is about denying the existence of god, and agnosticism is about not believing or disbelieving. It's just a matter of definition in the end, what actually matters is the meanings behind the words.

It's also not a clear line, you can for example have a slight belief that god doesn't exist, but this belief is superseded by the rational thought that there is no evidence of it nor the opposite, and ending up effectively having an agnostic conclusion. That's why words like that are always clearer when explained further in details.