Simply put, it’s the list of your most essential tasks from different projects. You choose which tasks become your priorities and then add them to this task list.
The task appears on this list only when you mark it with a star.
If you create a new task in your Priority view, its location will be automatically set to Single tasks. This is because the Priority view isn’t an end location for a task by itself. It gathers tasks that you’ve marked with a star from other lists.