The iframe module that allows to add iframes as a field fials for safety reasons. The browsers see this strange matter as a threat. Sites that you want to show as iframe must give permission to do so. Eg. Youtube and Google (calendar, maps, ..) do this otherwise you'd never be able to integrate their content on a site.
Sometimes it is useful to use iframes. So as a teacher i want to show, on a page of the site, a glimpse of the educational sites I work with. (I am of course administrator of both sites and I know what I do.)
This module that you just have to activate, authorizes the source site to be displayed on the target site. No warnings of the browsers even with https. (Below is a print and therefore does work as iframe ..)