Every time you edit a page, Wikked stores a new revision of the page. The series of revisions, going all the way back to the creation of a page, constitute the history of a page.
All the changes across all pages make up for the history of the wiki.
You can generally see the history of a page by clicking the "History" entry in the sidebar. The history will be a list of revisions, where each revision is shown as:
An identifier for the revision, which is how the source control management system (SCM) knows about this revision. It depends on the SCM you chose when creating your wiki -- with Mercurial it's going to be a revision hash, and with Git it's going to be a commit SHA.
The date when the revision occured.
The author of the change. If the person who created or edited the page wasn't logged in, the IP address will be stored instead.
A comment that the author may or may not have written when they created or edited the page.
The ability to show the changes that the revision did to the page (i.e. the difference between the page's prevision revision and the selected one), and the ability to show the changes between two arbitrary revisions (i.e. selecting two revision and showing the difference between them).
The revision identifier will usually be a clickable link in a page's history, which shows you what the page's raw text looked like at that point in time.
There is also a button at the bottom of that page which lets you revert the page to that revision. This will create a new revision which effectively brings that page's text back to how it was.
You can look at the entire wiki's history in the special History page.
That page will show you a similar list of revisions as with page histories, but with a different layout, because each revision will now also list the pages that were modified in each revision. Although someone editing pages through Wikked's web interface will only be able to change one page at a time, what sets Wikked apart from some other wikis is that pages are simple text files stored in a standard SCM like Git or Mercurial, so it's possible for some changes to have been done outside of Wikked (using your text editor of choice), across several files at the same time. See Editing Pages.