Can someone please explain the meaning of server side and the client side?

Answer 1

In simple words:
Client side means code that executes on the user's device like running a Javascript code in your web browser.

Server side means code that executes on the server like running php, nodejs , ruby code on a web server.

More detailed info below:
https://www.techopedia.com/definition/439/client-side
https://www.techopedia.com/definition/439/server-side

Answer 2

Krishna is correct. a good example of this would be string files used in MMOs. the actual text for the file is stored client-side when the server sends a reference to that text it is loaded from the clients end and displayed in their UI. this enables large menus, Quest descriptions and other types of heavy Text data to be displayed almost instantaneously without the need to download it from the server.

this is just one example of why it's important to have the two different software systems working together, client and server.