The term "self-organizing" is used in the Scrum Guide to describe a key characteristic of the Scrum Team. In general, it means that the teams are expected to coordinate and make decisions around the work. An alternative term is self-managing.
Self-managing means the team decides how much work they will take into the sprint, how they will go about doing the work, and which of them will do the work. Using the Product Backlog, the Product Owner decides what that work is and when it might get done by prioritizing the items.
