What is Custom Script and how it works

What is the purpose of custom script and how it works in erp next

IT allows you to add your own custom script on top of existing core scripts, and the best of all is, it doesn’t get replaced during a system upgrade.

https://erpnext.org/docs/user/manual/en/customize-erpnext/custom-scripts/custom-script-examples

For that you must have system manager role