Priority:- When a task should be worked upon or when a bug should be fixed.
Severity:- What is the effect if this problem is not operate upon now/a defect is not fixed now.

High priority and High severity:-
you are testing a website and its login functionality does not work. Then this issue must be fixed now to go ahead and if not fixed now then it blocks entire work.

High Priority low severity:-
A validation message to be shown to user in case of wrong credentials are submitted. The message is not useful. Though resolving this is not a big job but influence at customer end is important.

Low priority high severity:-
One of the module is not functioning the way it is expected and customer is fine to defer its release.

Low priority low severity:-
Generally cosmetic(Spelling Mistakes, Alignment and color changes) issues come under this section. Defect which doesn't affect on the functionality of the application