A portal is a system component an app can invoke. Through this portal, the app can get access to single elements, without needing any permissions.
Portals are great and need to be used EVERYWHERE. On Linux with Flatpak, the state is pretty horrible. But its getting better, slowly.
Technically: a portal links external files to the apps internal container. The files that every app can use. And by that any app can send what you want, without needing file permissions.
Its bad if apps on purpose refuse to use that, like Whatsapp and signal do