The scrum of scrums is an extra scrum event used when multiple scrum teams are collaborating together on a single product.
The scrum of scrums follows a similar pattern to the daily scrum session. The session allows the teams to update each other on what has been done, what obstacles have been encountered, and what to do next.
The scrum of scrums allows those teams to stay in sync and account for dependencies that bridge across teams. When facilitated well with healthy teams the scrum of scrum can even create collective ownership you see in self organizing teams.
If multiple scrum teams are collaborating on a single product then ideally both teams care more about it the product as a whole succeeds versus just caring if they did their part. The transparency, evaluation and adaptation from the scrum or scrums can make this possible.
To learn more explore the most common terms in a Scrum glossary.