Each task/note can specify which group(s) have permission to view/edit this task/note. Adding a group to a task allows all members of that group to see/edit the task according to their User Permissions.
When you are creating a new task, the task groups are initially set to what you have specified in your Preferences.
You can override them on a per-task basis if you like.
If you don't set any task groups, only the task owner will be able to see the task.
Why this is important
Many people interact with different groups within a larger organization. They may have certain tasks that are appropriate for viewing by one group (Management) but not by another group (Staff). This allows a single user to work with both groups, but not clutter each group's view of what needs to be done with tasks they don't need to worry about.