The meanings of prerelease and build meta tags in SemVer strings is entirely up to the publisher. Always consult the publisher’s documentation before acting on tag content. As @lemieuxster points out however, “rc” is likely an acronym for Release Candidate.
In the absence of publisher documentation, you can not make assumptions about meanings of any prerelease tag. The spec defines precedence based entirely on the tag’s ASCII code points and the number of fields: 1.0.0-airdale < 1.0.0-airdale.dog < 1.0.0-boring < 1.0.0-zed. They don’t have to have any implied meaning for the sort algorithms to do their job. There’s nothing special about “-alpha” unless the publisher documents what it means.