Trouble understanding torsion

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I'm reading this paper. In definition 6.1.2 it is mentioned that $H_n(G;A) \cong \text{Tor}_n^{\mathbb{Z}G}(\mathbb{Z}, A)$ (for all $n=0, 1, 2, ...$). What exactly is meant by the right-hand side? I'm familiar with torsion in other contexts, but I'm not sure how to interpret the superscript $\mathbb{Z}G$.