Hi, client scripts are used to alter data and\or the visibility of data in the UI (browser).
If you want a better basic understanding of client script usage you should review the vendor documentation
https://docs.servicenow.com/
Register on the developer portal to spin up a free 'Personal Developer Instance' for testing\experimenting
https://developer.servicenow.com/
and then work through the free learning material available on the developer portal and other learning material available at https://nowlearning.service-now.com/
You can also browse\filter existing community forum questions and articles for those that contain client script related information.
