M1, M2, etc.. are milestone releases. Each milestone release includes newly developed functionality in AS. Milestone releases are mainly meant for developers who want the latest technologies.
CR is a release candidate. It's in preparation for the Final release.
The latest "stable" release is 5.1.0. If you wait for one more month then the latest stable would be 6.0.
Thanks Jai for the information
Also, stable releases use the designation GA, as in 5.1.0.GA.