I need to understand the concept of add_rewrite_rule() in wordpress in my case

I have a subdomain based website by wild card

I created a custom page called X.php

This custom page function is to get an id from the link and do it's function

The link should be like that :

x-a.domain/x.php?id=x-a

How to convert this link to

x-a.domain/x

Using add_rewrite_rule()