What is admin_devel module in drupal? THis module is separate from the devel module that gives ability to dpm() variables etc. What is it?
The admin_devel module actually comes with the admin_menu module.

It adds a debug function for javascript debugging (along the lines of dpm for php), which you can include and use.

It also adds a button you can press to rebuild system links, which is on the admin menu settings page.

This rebuilds menu item links that are handled by the drupal core system module.