The Index Page is the page shown when the root index / is accessed.

For each siteaccess, you can configure IndexPage, as in eZ Publish 4.x. Put the parameter index_page in your ezpublish.yml file, under the right siteaccess category.

ezpublish:
    system:
        mygreat_site:
            languages:
                - eng-US
            #The page to show when accessing IndexPage (/)
            index_page: /yourURIPage