Wiki pages are written in a simple, plain-text syntax that is extended with a few more plugins and a Math formulas, that allows structuring the page and pleasant formatting while retaining high readability of the page source. When you edit a page and save your changes, the previous content will not be lost. Instead it will be saved as an old revision of the page and can still be viewed or even restored.
Before you edit your first “real” page in the wiki, you should try out the possibilities here, in the playground. The playground is provided so that users can test things and play around to their heart's content. (A playground is created when installing DokuWiki, in the “playground” namespace. This page is periodically cleared, so you can make any changes you want.