What are the rules in CSS and what is the "at" used for

There are rules in CSS that do different things, but with them I saw the @ symbol used in several keywords, such as:

@import   
@media   
@keyframes  

and others, but I'm looking to find out what @ means , and how it is used or better, how it is profitable, modified or different from the rest of CSS.

I read the W3 section on at-rules , but it doesn’t explain clearly (I really read it, and I ended up even more confused) what @ means , and it is specifically used for.