A FOSS platform is a place for sharing knowledge, ideas and for working together. It is a feeling of togetherness. A platform might refer to a name under the umbrella of which people with similar interest gather up. It also might be an organization working for uniting like minded people. Platform concept in case of FOSS movements is necessary as FOSS movements are all about collaborations among common contributors.
FedoraProject as a Platform
FedoraProject can be seen as an example of platform. Under the brand name of Fedora people all over the world gather to work on betterment of FOSS technologies and collaborate with like minded people joining from all over the world.