Where is HTML, JavaScript, CSS, and URL encoding used for input and control characters before being displayed in the browser?

Question

Information Security

Good afternoon everyone!

A letter arrived from above on the security of sites and information systems.
One of the items was:

Encoding input and control HTML characters, JavaScript, CSS, and URLs before being displayed in the browser

Not very strong in information security, and Google did not give any special answers, and the information security service did not give anything to my question: "Show me an example where it is used or a link to a detailed description?"

In the end, I ask the GURU to help.

Has anyone faced such a task or the answer is on the surface, just the wording is leading in the wrong direction?

I would be grateful for any information.