if(preg_match('/(benchmark|sleep)/i', $id)) checks if the $id matches the stringsbenchmark or sleep (the i stands for case-insensitive).
In the context it's presented I'd say this makes no sense what so ever though... I'd rather do this, and be done with it:
$id = (int) $_GET['id'];
$result = mysql_query('SELECT id,name,pass FROM users WHERE id = '.$id);
Notice I cast the id to an int, so if it's anything else it should just end up being 0, which most likely doesn't match anything since id columns usually starts on 1 (from my experience anyways).