A rollback makes an earlier version of a site live again, instead of you undoing an edit by hand and hoping you remembered every change. It is only possible if the host actually keeps past versions to roll back to; a site served from a single folder of files has nothing to revert to.
Birta keeps the last ten published versions of every project. Rolling one back does not delete the version it replaces or change anything on the files on your own machine.
For what actually happens to the live address during and after a rollback, see update a live website without redeploying the whole site.