Question on an exercise involving the modular group

Question

I‘m working on an exercise of a book over modular forms (Henri Cohen, Modular Forms, a classical approach) and I’m confused by the frasing of the question: Show that the map $\gamma \mapsto \gamma$i induces a bijection from $SO_2(\mathbb{R})\setminus SL_2(\mathbb{R})$ to $\mathbb{H}$ , where $SO_2(\mathbb{R})$ is the group of rotations. I don‘t know what is ment by $SO_2(\mathbb{R})\setminus SL_2(\mathbb{R})$. I thought of a set difference, but I think this wouldn‘t make sense. In the same book this symbol was also used for a group action, but I don‘t know if this would make sense here. It would be nice if someone could explain the question to me. Thanks a lot, Philipp