A server-side language is the programming language (also known as a code or a script) used on the web server to produce the website.
There are a variety of server-side languages. Each has their own pros and cons, but they all perform the same tasks.
Read more about server-side languages in general:
You should have a basic understanding of the most popular server-side languages.
A back-end developer is skilled at least one server-side languages. Most back-end developers know at least two languages well.
Some front-end developers do not know any server-side languages at an expert level, and some front-end developers know multiple server-side languages very well.
All front-end developers eventually work with server-side languages, but rarely at a complex level.
Read through these links to gain a basic understanding of these popular server-side languages: