Most repositories, including RPMFusion, use cryptography to sign their packages which helps the package manger determine if the packages are unmodified (to make sure they have not been tampered with). Since you just installed RPMFusion, it is asking you to download the GPG key that will be used to check the signatures on the packages you install from that repository.
I just wanted to expand on your question a bit, for more information about how Fedora GPG keys and signatures work check out the link @vgaetera posted…
It is Fedora release key, not RPM Fusion, and this happens in every new installation.
Although I heard that it is supposed to be fixed in Fedora 2x release, the issue seems still relevant.
Off-topic to your actual question, but just FYI: As @vgaetera said, that’s the Fedora 30 signing key, and it would have come up even if you hadn’t added RPM Fusion. It was shown because you asked dnf to update the multimedia group, which is also a Fedora package group, and this was the first time you’ve used dnf to install Fedora packages on that system.
If you’d typed sudo dnf group update multimediawithout installing RPM Fusion’s repos, you would’ve gotten exactly the same thing when dnf updated some or all of these packages:
$ sudo dnf --disablerepo=rpmfusion\* group info multimedia
Group: Multimedia
Description: Audio/video framework common to desktops
Mandatory Packages:
PackageKit-gstreamer-plugin
alsa-plugins-pulseaudio
alsa-ucm
alsa-utils
gstreamer1-plugins-bad-free
gstreamer1-plugins-good
pulseaudio
pulseaudio-module-x11
pulseaudio-utils
Conditional Packages:
gstreamer-plugins-bad-free
gstreamer-plugins-espeak
gstreamer-plugins-good
With RPM Fusion, a few more are added:
$ sudo dnf --disablerepo=fedora --disablerepo=updates group info multimedia
Group: Multimedia
Description: Audio/video framework common to desktops
Mandatory Packages:
gstreamer1-libav
gstreamer1-plugins-bad-freeworld
gstreamer1-plugins-ugly
Conditional Packages:
chromium-libs-media-freeworld
ffmpeg-libs
gstreamer-ffmpeg
gstreamer-plugins-ugly
qt5-qtwebengine-freeworld
(And if you install any of those, you should expect to get another key import prompt much like the previous one, but this time for the RPM Fusion Free package signing key.)