The term comes from the days of ARPANET, the predecessor to the internet, where the researchers would basically just throw ideas out there to, well, make a request for comments from the other researchers on the project. They could be about pretty much anything and were not very formal at the time. If you go read them, it’s pretty comical how informal they were.
Now, there are more standards about what goes in RFC’s and you can’t get an RFC published until you have met strict guidelines and have done extensive research. They are pretty much reserved for well researched network standards that have been approved by the IETF.