What is server side language? How it works?

Question

Please clear my mind with e.g

Answer 1

Examples are so many. E-shoping, e-banking, news servers, chats, e-mail in web page, or here learning on Sololearn, or such this page with form with question and answers.
So,, everything in web page content what could be changed upon anyone requirement (e.g. the client needs to view only his bill which is generated from his bank evidence)
There are two types of programming these changes in webpages -
client side programming (Java script) just for simplest programming - right in client's browser
a
server side programming - for huge programming, and especially for getting data from external data sources (like dadabase, or archives of pictures, videos, articles, and so
generated the selected into web page }This server side programming is a php for example