Menu_slug used for creating options page

Question

I am following this page on designing options menus: http://codex.wordpress.org/Administration_Menus

What is the significance of the menu_slug ?

I ask because when I change this value, the options page changes to

You do not have sufficient permissions to access this page.

If I change it back to what it was, it's fine.

---EDIT---15/8/2011

This is the code I was changing:

function highlighter_menu_() {
    add_options_page('Highlighter pen options', 'Highlighter Pen', 'manage_options', 'my-unique-identifier', 'highlighter_plugin_options');
}

and changed it to

function highlighter_menu_() {
    add_options_page('Highlighter pen options', 'Highlighter Pen', 'manage_options', 'a-unique-identifier', 'highlighter_plugin_options');
}

Answer 1

I have found the issue. I was making changes to the plug-in code and transferring it directly (via ftp). I forgot to deactivate and activate the plug-in. Doh!