This is usually done by routing all requests to a single entry point (a file that executes different code based on the request), with the following rules:

# Redirect everything that doesn't match a directory or file to index.php
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule .* index.php [L]

The file then compares the request($_SERVER["REQUEST_URI"]) with a list of routes - patterns that match the request to a controller action (in an MVC application) or other actions Map execution paths. Frameworks often include a route that can infer controllers and actions from the request itself, as a backup route.

A simple example:

<?php

// Define a couple of simple actions
class Home {
    public function GET() { return 'Homepage'; }
}

class About {
    public function GET() { return 'About page'; }
}

// Mapping of request pattern (URL) to action classes (above)
$routes = array(
    '/' => 'Home',
    '/about' => 'About'
);

// Match the request to a route (find the first matching URL in routes)
$request = '/' . trim($_SERVER['REQUEST_URI'], '/');
$route = null;
foreach ($routes as $pattern => $class) {
    if ($pattern == $request) {
        $route = $class;
        break;
    }
}

// If no route matched, or class for route not found (404)
if (is_null($route) || !class_exists($route)) {
    header('HTTP/1.1 404 Not Found');
    echo 'Page not found';
    exit(1);
}

// If method not found in action class, send a 405 (e.g. Home::POST())
if (!method_exists($route, $_SERVER["REQUEST_METHOD"])) {
    header('HTTP/1.1 405 Method not allowed');
    echo 'Method not allowed';
    exit(1);
}

// Otherwise, return the result of the action
$action = new $route;
$result = call_user_func(array($action, $_SERVER["REQUEST_METHOD"]));
echo $result;

In conjunction with the first configuration, this is a simple script that allows you to use URLs like domain.com/about. Hope this helps you understand what's going on here.