A ref is anything considered a "reference" (or "ref") by Git ([linky](http://git-scm.com/book/en/Git-Internals-Git-References)).  Examples include a tag, branch, commit SHA, or stash, so an example command would be: `yogi perf --ref v3.10.0 --ref dev-master --ref c0d88f --ref 4016d0644473ab30d59e5f75f7ffa08fcb2ecedb`, and it would run a comparison against all those refs.

But, in working with people, most don't know what a git "ref" is, so I'm certainly open to ideas.