The “Is priority sort dependant” option can be found in the Project settings in the Administration menu. This feature, when checked makes it so that whenever a priority rating is given to User Stories, they will be automatically sorted according to their priority, on the Backlog board. E.g. User Stories with 5 stars of priority will appear above stories with 2 Stars of priority.
The sorting can be changed between ascending and descending orders by clicking the Column title. If it is left unchecked, User Stories will not be sorted according to priority.
Below is an example of what the Backlog is like when the option is checked:
Below is an example of the the Backlog when the option is unchecked: