The Expedite swimlane comes by default in new boards and it will show the issues with the highest priority (based on the JQL priority = Highest).

Board administrators can configure those options under Board Settings > Swimlanes, but it will be there when starting a board from scratch.

This Swimlane will only show up in a board, once their criteria is matched, which means, you should only see them once an issue is set with the highest priority.

It would be a great improvement to somehow disable this option and make sure Swimlanes will only exist when configured.