Request for Comments (RFC) are documents that outline a proposed feature to be added to Mun. They are added in a pull request where the community is able to comment on the proposal. This is the time to gather feedback, support and reach a consensus.
Eventually, somebody on the Mun core team will either accept the RFC by merging the pull request, at which point the RFC is 'active', or reject it by closing the pull request.
Once an RFC becomes active then authors may implement it and submit the feature as a pull request to the Mun repository. An active RFC does not mean the feature will ultimately be merged. It means all the major stakeholders have agreed to the feature and are amenable to merging it.
Modifications to active RFCs can be done in followup PR's.