These are variables whose values are automatically filled in before your script starts to execute. You can use them anywhere in your scripts. See the list at [php.net...]
If you plan to write a lot of PHP code, you can download the documentation from here: [php.net...] In Windows, I use the .chm Help file with user notes. In Linux, I use the "many HTML files" version. I find both very usable.
A FYI, $_SERVER is one of the PHP superglobals (not functions, $_SERVER is a global array available to any part of a program,) and in other languages are called environment variables. They are extracted, and changed based on, the server software environment.