M1 means Milestone 1, it’s a release name, like beta or alpha.
RC means Release Candidate.
A milestone means that the application got a huge improvement from the todo list.
A release candidate is a release that can be the final release unless some major bugs are found.
To quote the ASF:
Releases that only represent a project milestone and are intended only
for bleeding-edge developers working outside the project are called
“alpha”.
Resources :
- Wikipedia – Software release life cycle
- ASF release types