SEO friendly links if activated in the config will replace your urls with human readable ones that can be easily remembered and search engines will like a bit more.
For an examle if I would access about without SEO Friendly Links I would use:
http://jcore.net?menuid=1
while with SEO Friendly Links activated this looks like:
http://jcore.net/about
For now thre things are converted to SEO friendly links, Languages, MenuItems, Posts. For e.g. a post with the path about-us in the menu about in the language en would looklike this:
http://jcore.net/en/about/about-us
By default jCore's SEO friendly links are activated as nowadays each server supports .htaccess file by default so it should work right away but if doesn't you can always deactivate this feature from your config till you find the problem as these links make your site a lot more attractive to the search engines.
With SEO friendly links you not just have a more attractive site for the search engines but you also have a more user firendly website where people can navigate more easily and can share your urls without having to write them down.