This is displayed at the main approval panel when the version of the file currently being viewed is not approved, and another, approved version of the file exists (if several approved versions exist, the most recent is referenced in this context).
If you are viewing an offer, Open latest version is a button, and you can click it to open the latest approved version of the offer in a new Offer Designer instance (note that it is not possible to open a previous version of an interaction, and, as such, a non-clickable icon is shown instead of a button in the Interaction Designer). If the version of the offer in question is already displayed, it receives the focus.