It's almost like a little word processor and makes creating neatly formatted texts convenient and simple. This video shows the highlights of the WYSIWYG editor.
Now you have got a good impression what's possible. Everything is very accessible through the editor's menu bar at the top. But it gets even more efficient with keyboard shortcuts.
These shortcuts will trigger autocomplete, which will prompt you with a list of suggestions to choose from to complete your entry. There are way more shortcuts that work in the editor, have a look at Atlassian's online documentation about the editor or click the help icon on the editor toolbar.
Autoformatting is a great feature for experienced users who love Wiki markup. Have a look at Atlassian's demonstration video how this works.
You can disable the editor's autocomplete and autoformatting functions, by editing the settings in your user profile, like this: First, click on your profile picture and then choose Settings. Second, click on Editor under Your Settings in panel on the the left-hand side.