De acordo com a página inicial :
RPM Fusion provides software that the Fedora Project or Red Hat doesn't want to ship. That software is provided as precompiled RPMs for all current Fedora versions and Red Hat Enterprise Linux 5 and 6; you can use the RPM Fusion repositories with tools like yum and PackageKit.
De acordo com as Perguntas frequentes :
RPM Fusion is a repository of add-on packages for Fedora and EL+EPEL maintained by a group of volunteers. RPM Fusion is not a standalone repository, but an extension of Fedora. RPM Fusion distributes packages that have been deemed unacceptable to Fedora.
A principal razão de sua existência é dada aqui :
Fedora is effectively bound by the same legal restrictions as Red Hat, as a US company, is bound by. This means in particular that software encumbered with US patents cannot be included in Fedora.
Em outras palavras, é um repositório de pacotes de terceiros para as distribuições Fedora e RHEL destinadas a distribuir software que não pode ser incluído no repositório oficial devido a considerações legais.