server-side=your app will work on server, when it works will return result. client asks from server, server execute commands e.g. get client list from a database.

client-side=play with browser (chrome or edge, firefox or etc) by using JS (or similar scripts). you can run scripts by using browser. about anything you are allowed by browser ofcourse. e.g. change content of textbox, change color of a span etc

briefly;

server-side=get data which is located on server by using a script page

client-side=play with client s browser