Manual:Load.php

load.php is a script for the front-end to hook into for loading ResourceLoader modules.

Styles not displaying
After they updated to MediaWiki 1.18 or newer, many users had the problem that their wiki showed up without styles. CSS styles were not applied; what they saw instead looks like a trimmed down version. All styles are browser defaults; no special styles are applied.

Solution:

The problem in most cases is caused by the fact that requests to load.php are redirected to the MediaWiki main PHP file, which finally returns a MediaWiki page. The expected output however are CSS styles.

To fix the issue you have to add a RewriteRule in the file .htaccess. Change your .htaccess file to make sure that calls to load.php (and to api.php btw.) are not rewritten.

RewriteEngine On RewriteRule ^(api|load)\.php - [L] ...

After that load.php should return the right contents and your wiki should be styled.

Details
First it checks the PHP version, which must be 5.2.3. or higher, and it includes WebStart.php. Next, it creates a object and calls its  method, which outputs the specified resource content, including a content-type header.