We just met it while creating your own homepage: the text-based editor of MoinMoin. You are, of course, not limited to only using it for that, because you can also use it to change any other page in the wiki.
You can do that simply by clicking on edit or edit (text) in the navigation area and MoinMoin will bring you to the text editor for the current page.
You can't mess anything up, because the previous version of the page is always saved and can be restored if needed.
It is important to frequently use the preview function while editing a page, since using it will tell MoinMoin to create a backup of the editor's current content.
When you're done, you should enter an appropriate comment (which will then appear on RecentChanges and in the history of the page) and click save.
Should there be two people editing a page at the same time, the first person can save without problems. The second person will get a warning and should merge the first person's changes with his own. If you want to edit a page on your own, you should state that on the top of the page, so other people will read it.
You should not go back to the editor page with the "back" button of the browser after editing, because this can lead to strange editing conflicts. Just use MoinMoin's edit button again.
If you do not want to save, but instead discard the changes, you should click cancel, so that MoinMoin knows that you are done editing the page.
Further information can be found at HelpOnEditing.