Release version numbers are a part of the version name that indicates what the last release was. For stable release builds, the version string only contains a release version, but version names for development and release candidate builds include other information such as the release candidate revision and the development build number.
MultiMC's release version numbers consist of three numbers, the major revision, minor revision, and hotfix number. The "5" in MultiMC 5 is not a part of MultiMC's version numbers, it is simply part of the name. It's MultiMC 5 version 0.3.1, not MultiMC version 5.0.3.1. The major revision number increments when major features are added or major code changes are made. The minor version is incremented whenever smaller features or bugfixes are made. The hotfix number is only incremented when hotfixes are made and is usually excluded unless it is greater than zero.
The version name is the entire version identifier in string form. For example, "0.3.1-dev142", "0.3-rc3", or "0.3.2".