I'd recommend you check out those books because they explain it far better than I or anyone else could hope to in a single comment, but it essentially comes down to the capitalist/worker divide. The capitalist will always be on top, and the worker will always be subservient to their interests. That is how it is hierarchical, because there will always be the capitalist class imposing it's will on the working class, and so anarchists and communists hope to abolish the class system. However the two groups disagree massively on how to achieve that abolition.