this has nothing to do with PHP necessarily but the HTTP spec allows for the client to send name/value pairs passed on the URL query string like so:
http://www.example.com/index.php?name1=value1&name2=value2
then the webserver can then receive these pairs as:
name1 = value1
name2 = value2
PHP will automagically put thes into the $_GET superglobal array