In a Scrum of Scrums (SoS), each Scrum team has a designated person that represents that team in a new virtual Scrum team. This team does all activities of a normal Scrum team, albeit in a scaled way (so daily stand-up, refinement, planning, review and retrospective).
The goal of an SoS refinement could be to divide user stories over teams. For example, consider a SoS of two people (representatives from two teams), a product owner and a scrum master (both responsible for the two teams). The two people, the product owner and the scrum master decide together which stories will be put on what team's backlog, using the combined knowledge of all four people to make this decision.