This is a feature in the Customer Support add-on that lets you prioritise your tickets based on your rules.
In Settings > Customer Support > Ticket fields and admin can select if priority should be enabled in tickets or not.
Priority best practise
Priority levels are defined by Upsales; No priority, Low, Medium, High, and Critical. You probably have your own way of prioritising but if you don't, a good starting point is to weigh urgency versus Impact.
How priority affects tickets
The tickets are sorted based on their priority in the different ticket lists across Upsales. The highest prioritised tickets will be shown at the top of the list.