This file "bootstraps" the EE environment, so you use the EE "super object" in EE2, or the EE global objects in EE1.

Useful for little one off scripts, or cron jobs that need access to the EE environment.

Example:

<?php
$system_path = '/path/to/system/folder';
include '/path/to/bootstrap.php';

$EE = get_instance(); // We haven't been using PHP4 for years!
$DB = $EE->db;

require_once APPPATH.'libraries/Template'.EXT;
$EE->TMPL = new EE_Template();
...

Note: EE2 seems to kill some superglobals, so you might want to save these and repopulate them after the bootstrap file is included.