By default apps are set to "auto publish".
You can change this for a deployed app via File > App Settings.
"published" just means that the app was visible to its users. Versions not marked published are only visible to the app designer.
So, by default, every change is "published" and if it is just a single person building an app, this can be ignored if your app is not yet deployed to a bunch of users. This is a new feature that rolled out in November 2015, so full documentation has yet to fully catch up.
ParaSQL in design mode auto-saves as you build your app.
So "undo" means reverting to the last saved version (basically as soon as you stop typing for a few seconds, a "version" is saved).
ParaSQL keeps at least the last 15 non-published PLUS at least the last 15 published versions of your app. So you have many "undo" choices.